UK Security Services records at the National Archive
The UK National Archives has just put up a database with materials from the Security Services:
“Some of these files were kept about individuals; others were gathered on groups in which the Security Service took an interest. The files contain information about communists, Soviet intelligence agents and officers, right-wing extremists, Italian espionage activities and suspected agents, Czech refugees, suspected spies, pacifists, German agents and intelligence officers.
The majority of these files are from 1939-45, but there are a considerable number from the inter- and post-war periods.”
Note, not every result is freely available – most items are only available for a fee and there is currently no way to limit your results to free materials. That being said, the range of materials in the collection is impressive: spy diaries, Hitler’s passport, and files on prominent UK communists and eminent physicists and more.