On the Record: MI5: Official Secrets

Chloe Lee: James Bond, George Smiley, mysterious gadgets and coded messages. Spy fiction has captivated our imagination for decades, but the truth behind Britain's security service is even more fascinating than these high-stakes capers.

Here at The National Archives, we hold the real files, declassified documents that reveal the actual operations successes and sometimes failures of MI5 since its creation in 1909 in fact, we're about to showcase many of these files, plus spy gadgets and other objects in a new exhibition that opens at our site in queue on April 5th.

I'm Chloe Lee, a record specialist at The National Archives. I also host our podcast On The Record at The National Archives, uncovering the past through stories of everyday people.

In this episode, as we announce the new exhibition, I want to explore the remarkable history of MI5 through true stories that go beyond anything dreamed up by Ian Fleming or John le Carré. A female agent who infiltrated communist circles in the 1930s and a Danish double agent whose deception during the Second World War saved countless allied lives.

To guide me through these now declassified files, I've invited two guests who have unravelled the real-life mysteries within our collections.

In this episode, we've got contributions from Gill Bennett, a historian who specialises in secret intelligence and former Chief Historian at the Foreign Office. Gill will be setting the context.

And in the studio, I'll be speaking to Mark Dunton, Principal Records Specialist at The National Archives. Welcome to the studio, Mark.

Mark Dunton: Thank you. Chloe, it's great to be here.

Chloe: I'm really excited about our conversation today.

Mark: Yes, indeed.

Chloe: So I need to know exactly what MI5 is and how it came into being. So let's first hear from Gill explaining that context.

Gill Bennett: MI5 grew out of the creation of the Secret Service Bureau in 1909 and that was a body set up by a committee of imperial defence in expectation there might be a war with Germany. So the body was set up to look at espionage against the United Kingdom. It started off doing foreign and domestic espionage, but within a year, in 1910 it split into two. MI5, the Domestic Intelligence Bureau, and what was then called MI1c, and later became the Secret Intelligence Service, the overseas intelligence agency. I should point out that MI stands merely for ‘military intelligence’. And of course, the domestic side of it is worried about, for example, trying to find where there are German spies in this country. When war does break out, that is one of the principal focuses of MI5 work, and they were extremely successful in detecting German spies.

But the important contextual point is that the First World War itself was a tremendous technological catalyst for developments in both domestic and foreign intelligence, and indeed (we haven't got to that yet) with signals intelligence. So lots of the early cases by MI5 were very formative for what happened later, they learned a lot. They developed new techniques about listening, intersecting mail, censorship, detecting spies, about all kinds of things which were going to be, actually, the foundation of their work for many years to come.

So by the end of the First World War, MI5, like all the intelligence bodies had grown hugely. Started off very small and ended up really quite large, and it had a very successful war.

Chloe: It's such a fascinating context. Now, Mark, is there a case from our records that demonstrates MI5 growing capabilities in the First World War?

Mark: Well, yes indeed, there's the Carl Muller case. Now, Carl Muller, he came over to England in 1915 with a group of refugees. He claimed to be Russian, but he was in fact German, and he claimed to be a shipping broker, but in fact, he was moving around the country and reporting on information such as troop movements.

Chloe: I see, I see.

Mark: Yes, he was sending letters to German intelligence in Rotterdam. These letters seem to be innocuous, but he was actually using invisible writing, using lemon juice between the lines, and giving them various info about as I say, items like troop movements, resources, that sort of thing.

The letters were intercepted by the postal censorship department, which was working with MI5. And what they did, was they passed a warm iron over the letter because they were suspicious about it. The warm iron then the secret writing in lemon juice emerged, yes, indeed. And so the authorities started looking for Muller. He did have an accomplice. He was a bit of a stooge in all this, a guy called John Hahn, who was a disgruntled German baker based in Deptford. The trail originally led there, but then, they found Carl Muller in his lodgings, I think, in Bloomsbury, and when he was arrested, they found a lemon in his dressing table drawer. And they said, why have you got that lemon? And he just pointed to his teeth and said, you know, sort of saying my teeth, as if he was using it to clean his teeth. And rather unsurprisingly, they didn't actually believe him.

Chloe: So whilst they uncovered the invisible ink, they didn't necessarily know how he was making the invisible ink. That wasn't until they discovered this lemon.

Mark: Yeah. I mean, the lemon was kind of, in a way, real proof of guilt, really, yes, because they already knew that lemon juice was being used, and in a way, it's one of the things that helped to convict him. So he was tried, found guilty of espionage and actually executed by firing squad at the Tower of London.

Chloe: So this is a real example of early forensics. Can we say that?

Mark: This case shows how amazingly effective the postal censorship department was. It shows the increasing use of forensics at this time. Because they even, have a specialist who even analyses the cellular matter on the pen nib, you know, and analyses it and says, this is lemon juice. You know, it's all very precisely done. And the other thing is, is the deception angle, because MI5 pretended that Muller was still alive after his execution, still sending messages to his German masters. And this was a forerunner of the double cross operation in the Second World War. And the other thing I must just tell you is that they pretended to be Muller. They were asking for more funds. Germany send more funds to us. Germany did send more funds, which MI5 used to buy a car, which they then christened The Muller, although apparently, the Treasury reprimanded them for unorthodox use of expenditure.

Chloe: And so we've got the lemon in the exhibition?

Mark Dunton: We have we've got the very lemon that was found in his dressing table drawer in the exhibition. It's black and shrivelled and compressed, but it is the lemon, and it forms part of the archive.

Chloe: Let's move on to the interwar period. We can hear from Gill on MI5’s focus during that time.

Gill: The focus of MI5 during the interwar years, between 1918 and 1939 was like all of British intelligence against the Soviet Union following the Bolshevik Revolution of 1917 that had brought a Bolshevik government into power in Russia. The perception of the Soviet Union as a hostile threat and employing espionage and sabotage against Britain, it was not a red under-the-bed scare. They really were doing that. And although during the 1930s MI5 and all the agencies would gradually move their focus as the Nazi threat grew in most of these years, the enemy is the Soviet Union and its extensive espionage threat.

However, after the First World War, MI5, like all the rest of British intelligence, was starved of money. It was cut down. The country was in a very bad way economically. The agency was cut back. Lots of people who had worked for it went back to their normal jobs after the war, and so you were operating, at least initially, with very small resources to try and do a very big job.

Chloe: It's also important to know whether the threat from Communism was the only concern during this time.

Gill: During the 1930s Britain is actually facing a number of threats all at the same time. So obviously there's the Soviet threat, but there's also the rising threat of rearming Nazi Germany, which after 1936 was also working with Italy and with Japan. So there are threats on all sides, all of which pose a threat to domestic espionage for MI5, as well as for the other agencies. The threat does shift, very largely towards Germany during the 1930s and MI5 had to really shift its focus while still being underfunded and understaffed and under-resourced. So it was definitely a challenge to meet the threats. Of course, they were worried about, still, worried about the threat of German spies in Britain. They were not as worried as they should have been. But of course, they didn't know this at the time that, of course, we now know, during the 1930s a number of important Soviet spies were recruited in the UK, including the ones we think of as The Cambridge Five. So it'll be Burgess, Maclean Cairncross, Blunt and Philby.

Chloe: Now we have that background Mark, let's get into the fascinating stories of Maxwell Knight and Olga Gray. Can you tell me about them?

Mark: Yes, yes, indeed. So you have Charles Maxwell Knight. Now, I think he was rather an eccentric character, but he was a legendary spymaster, particularly busy in the 1930s and he liked to use agents to infiltrate what was seen as subversive political organisations. It was a sort of softly, softly approach. They would be the ears and eyes looking out for vital information. He was certainly keen on building relationships with his agents rather than just giving orders. Now, the thing about Knight was, that he was particularly keen on the use of female agents because he thought that women had a sort of natural guile, and some of the language that he used to talk about this would really jar with our modern sensibilities. But in some ways you could, you could say he was quite progressive. Most people at this time would take the view, perhaps that a woman's place is in the home kind of thing.

Chloe: So maybe he saw women as particularly sharp tools when it came to building relationships with their targets, right?

Mark: Yes I think he did. And you know, so I think in some ways, he thought women had an advantage over men in this respect, as agents.

Chloe: And so is Olga Gray an example of that?

Mark: Yes, very much so. So Olga Gray, was in the early 1930s she was about 25 she was a trained commercial secretary, and she sort of, she did this sort of slow infiltration into communist organisations, including the Communist Party of Great Britain, and eventually, she became the leader Harry Politt’s Secretary.

Chloe: Wow. So really at the ear of leaders.

Mark: That’s right. So she was there, you know, listening, reporting back. I mean, there was a very strong fear of communism in this period. And MI5 was concerned for a long time about the Communist Party of Great Britain, perhaps rather too much. When MI5 looks back on this history over the whole period, they think maybe there was a bit too much concentration on this organisation. You know, for her, the files, I think, indicate that at times, all of this was quite a strain, and took its toll on her personally.

Chloe: You're holding two different identities at once, aren't you?

Mark: You are, that's right. And they sent her on a mission to India, for example. I think there were times when she felt like giving up. But the thing about Olga Gray is that she helped to break up a major spy ring, which shows the Soviet threat. This was the Woolwich Arsenal spy ring. There was a character that she was introduced to by Harry Pollitt, I think, called Percy Glading. And he was, he was very much trying to get defence secrets out of Woolwich Arsenal (where he used to work) and passing them to the Soviet Union. And Olga Gray foiled, it's largely down to her actually, that this was foiled.

Chloe: And that’s all because she was a trusted person, right?

Mark: Exactly, they did trust her. Percy Glading trusted her, and she gave evidence under a code name, Miss X. So she appeared in court using this code name. The press at the time loved it, because, of course, this was something new. This was something very exciting. You know an attractive woman who was a spy for us if you like…

Chloe: So Olga was monitoring these communist activities. She was feeding back. but we also heard from Gill about this growing threat from Hitler's Germany and a homegrown British fascism. Can you tell us a bit more about that Mark?

Mark: Yes, certainly. So I think MI5 when they look back on this history, admit that they were rather slow to appreciate the dangers posed by the rise of fascism, fascist groups at home. And I'm thinking particularly of Oswald Mosley and the British Union of Fascists. But when MI5 discovered that they were receiving foreign funding from Mussolini's Italy, they became more alarmed. And in the records, you see Vernon Kell (he was the first director general of MI5), you can see his reports about the British Union of Fascists, and he makes some very prescient comments about how they're copying Nazi methods very closely.

MI5 applied more than once for a Home Office warrant on Mosley. This meant that his correspondence would be intercepted, perhaps his phone calls tapped, but successive home secretaries blocked this. They seemed to regard Mosley as fundamentally patriotic, although in the end, Mosley was interned in 1940.

MI5 also tried to warn the government around the late 1930s about the real danger posed by Hitler's Germany and his expansionist aims, but the government were not prepared to listen to MI5 at this time.

Chloe: So let's move on to the Second World War, which seems to have been a pivotal period for MI5. Gill can explain how MI5 innovated during this time.

Gill: As with the First World War, the Second World War was a tremendous technological boost for MI5 and for all the intelligence agencies. In fact, their work became very important to each other, not just MI5 and what is now called MI6, and also of course, what was then called the Government Code and Cipher School, the signals intelligence agency, now GCHQ. Deception for all the agencies, and particularly MI5, is a major focus of the effort, and they all work together on this. And of course, MI5’s big achievement, which still remains absolutely astounding when you hear about it is the Double-Cross System. Which is basically turning German spies and getting them to work for Britain instead.

The other agencies played a part in that, It's important to remember. What Bletchley Park could provide, for example, from intercepts, which would let them know what the Germans did not know. For example, you might want to give your bogus agents what we call chicken feed, which means feeding them some information which the Germans would be interested in, and which indeed is of interest and might even be true, but is not giving away anything that you don't want to give away. But it is enough to entice them so that they believe what you're telling them.

To decide what could safely be revealed and what could mislead is very difficult. Important contributions were also made from the intelligence effort of countries that had been overrun, like Poland, who brought their intelligence services to London, as did some of the French. So they're all working together, but MI5 has an extremely good Second World War, and the double cross system is the pinnacle of its effort.

Chloe: Mark, who's an example of a double agent that folks might not have heard of before?

Mark: Well, there's Agent Tate. Now he was Danish born, and his real name was Wulf Schmidt, he landed by parachute on a spy mission for Germany in September 1940. He was quickly arrested and interrogated, and he was interrogated at Camp 020 on Ham Common, where suspected spies were interrogated. We have the actual transcripts of his interrogation, featuring in the exhibition Wulf Schmidt (Tate) becomes a double agent and the longest serving agent, wireless agent, in the Double-Cross System.

Chloe: So he begins to work for the British?

Mark: He does that's right, sending misleading information to the Germans and pretending that all is okay with his mission in Britain. The Germans trusted him completely, and he was awarded the Iron Cross, I think, in 1941.

Chloe: So utterly convincing?

Mark: Yes, indeed, you know, they really did. I think there was a quote from a senior German military officer who described him as ‘a pearl’, you know, ‘in our network’. This false information that Tate was sending the Germans made them, it made them make some significant mistakes in terms of military strategy. An example of this is that thanks to reports sent by Tate, German U-boat commanders were told you must stay clear of an area south of Ireland because there's a big minefield there that our agent has warned us about. And this had the effect of saving a really important convoy route into England. It probably saved countless lives. Another example is that he gave them false information about where the V1 and V2 weapons - where those flying bombs were landing in London. This forced them to change their trajectory so that they didn't fall so much in urban areas. Again, you know, I'm sure many, many lives were saved. So he did some heroic work for this country in the Second World War.

Chloe: And as part of the exhibition, we've produced some restaged audio of this interrogation. Tate's interrogation is based on the official documents that you've just referenced. Let's listen to a portion of that now:

Schmidt

I beg you to let me talk with the Danish Consul

, if possible.

Interrogator

No, no, you are a spy. What I want is the truth?

Schmidt

Yes, I will tell the truth or what I know. How can I do so? I mean, I came here, I fight in Denmark with a German. How can I how can you tell me?

Interrogator

How did you come here?

Schmidt

With a boat from Esbjerg

Interrogator

No, you didn't.

Schmidt

Yes.

Interrogator

No, you didn't. When did you come here?

Schmidt

On the 10th, on the night of the 10th and 11th of July.

Interrogator

That is a lie. I know that is a lie. Don't come here and tell me lies. You might just as well tell me the truth. We know a great deal about you. The sooner you make up your mind to tell me the truth, the better.

Chloe: So we know that Schmidt didn't eventually get turned into a spy for Britain. But Mark, what do these interrogation records reveal about MI5’s methods?

Mark: One of the officers who interrogated Tate was Captain Robin ‘Tin Eye’ Stephens, who had an ever-present monocle and a very fierce appearance. And he very much used methods of psychological pressure on these suspected spies. For example, the suspected spy would be made to stand. He would bark things at them, they would perhaps be deprived of sleep. But he forbade the use of physical violence because he believed it was counterproductive. But you can see a relentless determination, if you like, to crack the suspected spy and then see if perhaps they can be used against Germany. Turned into a double cross agent working for Britain against Germany. Not all of them were suitable for that purpose, and some were executed.

Choe: Moving into the Cold War period, Gill knows a lot about the main challenges facing MI5 in these years.

Gill: MI5, like the other agencies, suffered some of the same problems as they had after the First World War, and indeed, after the Cold War, the same because they're cut back. People always think, well, we don't need spies anymore, so they're short of resources. At the same time after the Second World War, what we now call the Cold War, really gets going, a split between East and West. You get more and more espionage and sabotage carried out by the Soviet Union and indeed by other countries within the Soviet influence. So it was a very busy time. You've also got a question of atomic espionage, several prominent atomic spies, such as Fuchs for example, and we now know, of course, that there were a number of spies, some of the Cambridge Five in the early 50s. Burgess and Maclean go to Moscow. It begins to become apparent that there have actually been significant Soviet agents operating within the British system. This obviously gives MI5 a big problem.

By the early 60s, there was a complete rush of what you might call ‘spy cases’, the Portland Spy Case, the George Blake case, a very important one in 1961, and then he escaped from prison. And of course, Philby, who had been suspected in the 50s and discharged from SIS but not prosecuted, does, in fact, finally defect and go to Moscow in 1963. The final defection of Philby is a really big blow to the British intelligence community as a whole. Now MI5, all of them, and MI5 and MI6 form together a joint committee to examine past operations. It's called the Fluency Committee, to try and see if they might have missed any spies.

Now, during the 60s, meanwhile, the Soviets are throwing a huge amount of effort into espionage at the Western world, the sort of thing that's been documented in the Mitrokhin Archives. And during the 60s, MI5 really can't cope. They cannot follow all the people in the UK that they think are Soviet spies. And that is what produces in 1971, what is still the single largest expulsion of intelligence officers, when the UK Government expelled 105 Russian intelligence officers, under Operation Foot.

Chloe: Those are some fascinating cases. Mark, do we have records about any of them?

Mark: Yes, we certainly do. In fact, there's a huge amount of documentation here, and the files are so rich, you know, they've got surveillance reports and transcripts of bugged telephone calls and transcripts from other listening devices. So the files are just amazing. Just in January this year, we had a major release of MI5 records, many of which concerned notorious Cambridge spies such as Kim Philby, Anthony Blunt and John Cairncross.

For example, we now have the confession statement from Kim Philby handed to an MI6 officer in Beirut when Philby was confronted about being a spy for the Soviet Union. Now it's very much a partial confession. Also in the files, we have a dialogue between Philby and the MI6 officer who's been sent out to Beirut to confront him, and Philby admits passing information to the Russians about the would-be Soviet defector Volkov.

Now Volkov, Constantine Volkov was an NKVD officer, and Russian intelligence officer, and back in 1945, he approached the British. This was in Istanbul Turkey, and he offered to defect. In fact, he said, If you can give me political asylum and some money, I can give you a lot of secrets about Soviet intelligence, and also, by the way, you have some spies operating at home. Some people in key positions at home are acting as agents for the Soviet Union. Philby got wind of this and tipped off the Russians. The Russians moved quickly. They kidnapped Volkov and his wife drugged them, took them to Moscow, brutally interrogated them, and then executed them. That was down to Philby. So Philby is admitting his culpability in this terrible business.

Chloe: So Philby was part of the Cambridge Five recruited by Russia in the 1930s. How damaging was the Cambridge Five operation to British security?

Mark: Very damaging I would say Chloe. Because you know that really, between them, they penetrated all the major institutions of British state security, and they were operating, you know, in the 30s, 40s, 50s, you know, so many secrets were passed to the Soviet Union, including nuclear sort of information, and many operations were compromised.

So, you know, there were scores of deaths as a result of some of the information passed to the Soviet Union, particularly by Philby. The other thing is that it was a huge resource drain on MI5 because, you know, for such a long time they were trying to find out, well, who are all the individuals in this Cambridge Five network.

Chloe: I guess they might not have necessarily known there were five, right? There must have been paranoia over other possible spies working inside intelligence services and the wider government.

Mark: That’s right. It led to a huge sort of distrust, and they realised we must have more exacting vetting procedures in future. There needs to be more careful checks carried out on people who are working in these high positions, so they did learn from it, but there was a terrible price that was paid for all their activities.

Chloe: This conversation for me Mark, it's really revealed that whilst there is this very glamorous kind of narrative about spies and spymasters, there's some real impact that really tears people's lives apart when it comes to this kind of history. I can imagine, there are lots of challenges, researching and writing about an organisation that, by its very nature, operates in secrecy. Mark, what would you say is the one big challenge that historians face when studying MI5?

Mark: Yes, well, I think the challenge really is that these cases of espionage are very complex and of course, the devil is in the detail. And so, you know, I'll give you an example. On the day after war broke out in August 1914 the Home Secretary, Reginald McKenna, announced in the House of Commons, that 21 spies and suspected spies had been arrested within the previous 24 hours.

But when you go and look into the files and what the files show, historians have found that actually there are some discrepancies, and they're quite significant. There are several different lists of these spies. There are real doubts that they were all arrested at the same time. And there are variations with the names. As a historian, you have to be forensic in looking at the records, you have to sift through the evidence and come to a judgment about things. But you know, these records from MI5 are so rich, and I love the spirit of openness in which MI5 have transferred these records to us and made them available for everyone to see.

Chloe: Finally, on April 5, The National Archives opens a new temporary exhibition about MI5. Mark, what makes this exhibition particularly significant?

Mark: Well, I think the exhibition is certainly groundbreaking because this is the first time that MI5 has collaborated with a public institution in telling its history. So again, I think there's a great spirit of openness there. The exhibition tells the human stories behind intelligence work, and I think, you know, there's a lot of food for thought, actually, from this exhibition, you've got real people making difficult choices, sometimes at great personal risk, such as Olga Gray, for example. And I think there's one or two other things I think, you know, MI5 is often mythologised in popular culture.

You know, often, if you just say ‘MI5’, it has a certain allure. And people might start thinking about James Bond, you know, that kind of fantasy world. But the exhibition, I think, is much more grounded in reality, and it's showing - so it's showing the reality of things, the historical fact, rather than the spy fiction. Although, of course, it's true. You know stories of espionage and the gadgets and equipment used by spies. This is fascinating stuff. It is intriguing, but we see the evolution of intelligence work over time as well. So really, I think people will find this very rewarding to see this exhibition.

Chloe: That's great. And can you share maybe three highlights that visitors can look forward to seeing at the exhibition?

Mark: Yes, well, it's always a difficult choice just picking three, but I think I can do this. So I would say seeing the actual lemon that was found in Carl Muller’s lodgings that helped to convict him of espionage, I think, you know, that's one fascinating item. Also we have actors who are reading a transcript of a bugged conversation in the Portland Spy Case, which has some almost unintentionally comic aspects about it. It's fascinating. And then also, there's a transcript which is derived from a listening device in the home of Klaus Fuchs, the atomic spy. And they can hear so much detail that they even talk about him stoking the fire in the grate you know, that's how closely they're monitoring him back in about 1950 - so those are three examples, which I hope will whet your appetite.

Chloe: Thank you, Mark, and thank you Gill. The exhibition, MI5: Official Secrets, runs from April 5th to September 28th 2025 at The National Archives in Kew.

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Finally, thank you to all our experts who contributed to this episode. This episode was written, edited, and produced by Tash Walker and Adam Zmith of Aunt Nell, for The National Archives. The audio of Agent Tate’s interrogation was produced by Chocolate Films Video Production.

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