You can learn more about this history of MI5 at The National Archives in Kew.
The National Archives website provides a full overview of the most noteworthy files released from April 2000 onwards.
We usually release files twice a year, generally each spring and autumn. It should be noted that some of the files we release are redacted to ensure the removal of sensitive information, and some individual documents may be retained for the same reason.
Security Service files in the National Archives cover many aspects of the Service's work. They fall into a number of distinct categories, which can be found in the National Archives catalogue under five "KV" headings (KV being the catalogue reference for the Security Service in general).
The headings are:
- KV1 - First World War historical reports and other papers. Includes some of the earliest Security Service files from the time of its establishment as the Secret Service Bureau, as well as documents relating to the roundup of German spies in World War I.
- KV 2 - personal files. Most of the files that we have released to date are in this category. They relate to specific individuals that were being investigated for various reasons concerning national security - generally relating to suspected involvement in espionage or subversion.
- KV 3 - subject files containing papers on various national security issues, such as German and Russian espionage activity in the UK.
- KV 4 - policy files dealing with a wide range of matters such as Security Service liaison with other agencies.
- KV 5 - organisation files relating to specific groups and organisations investigated for national security reasons.