More than 100 historical files have been released by the Security Service to the National Archives at Kew.
The 110 files demonstrate the breadth of the Service's work around the time of the Second World War countering diverse threats from Russian espionage to Zionist terrorism.
The documents include the remarkable wartime story of British fascist sympathisers, or 'Fifth Columnists', who believed they were passing military secrets to the Nazis.
In reality they were passing the information to an MI5 agent 'Jack King' who was posing as a representative of the Gestapo.
From today the files are available for members of the public to view. MI5 has now declassified and released more than 5,000 files to the National Archives.
For more information see The National Archives press release.