Security Service

The Security Service, also known as MI5, is the United Kingdom’s internal intelligence agency. It is responsible for protecting the country against covertly organised threats to national security, which include terrorism, espionage and the proliferation of weapons of mass destruction. In addition, the Security Service provides security advice to a range of government, and other organisations, helping reduce vulnerability to these threats.

The Security Service began life in 1909 as the domestic arm of the Secret Service Bureau. In 1916 it was incorporated into a new Directorate of Military Intelligence, assuming the name MI5. For much of the 20th Century, the key concerns of the Security Service, as it became known in 1931, were subversion and Soviet espionage.

As the Cold War threat diminished, the threat from terrorism, and especially international terrorism developed. Counter-espionage remains core work, but at a reduced level. International counter-terrorist work now takes up the largest proportion of Service resources and represents a notable change in priorities since the end of the Cold War.

Browse the files relating to the Security Service records in our catalogue

View document highlights to learn more about the Security Service

Go to the UK Security Service website to read more