Treasure was an effective double agent, but according to Masterman, the architect of the double-cross system, she was also ‘exceptionally temperamental and troublesome’. In conversations with her MI5 handler, Mary Sherer, Treasure revealed that she had let slip her double identity to an American soldier with whom she had an affair. She also threatened to stop working for MI5 unless they arranged for her beloved pet dog left in Spain to join her.

One month before D-Day, Treasure admitted she had agreed a secret signal with her Abwehr contact, Kliemann, so he would know if her transmissions were genuine. This meant that if another agent took over her transmissions, her cover would be blown, possibly putting at risk the whole network of double agents. In this document, Colonel Robertson, Sherer’s boss, records the angry meeting he had with Treasure in which he told her that her services were no longer required because her behaviour endangered the Allies.

The next day Sherer met with an upset Sergueiew, who suddenly said she would give Sherer the secret code agreed with Kliemann. She quickly wrote it down. This is that original note.

Sergueiew returned to France, but this was not the end of MI5’s troubles. In 1944 Robertson discovered that she intended to publish her memoirs, in which she referred to her MI5 handlers as ‘gangsters’ and refused to hide their identities. Robertson tried to talk Sergueiew into giving up the manuscript, but it was eventually published in 1968, with the title ‘Secret Service Rendered’.