Wartime spies have cover blown by MI5
The identities of operatives from the intelligence services are normally closely guarded, even after long periods of time have passed since their retirement.
However, an apparently innocuous file released by MI5 to the National Archives earlier this year has allowed a number of agents who operated during World War Two to be identified.
The information is contained in a schedule from the body's secret wartime interrogation centre, Camp 020, in which captured German agents were interrogated and "broken".
A series of names and date on the schedule can be cross-checked against other files where the names of agents have been blacked out, allowing their identities to be confirmed.
Stephen Dorril, an historian of the intelligence services, said: "It's really surprising that here is a file that can be used to identify agents.
"Its one of the 'no-no's' of the intelligence world that you don't identify agents."
Camp 020, based in Latchmere House in Middlesex, achieved unprecedented wartime successes and was crucial to detecting German spies entering Britain and turning them into double agents.
The camp was run by Lt Col.Robert 'Tin Eye' Stephens, so called because of a thick monocle worn over his right eye.
"Much like Stephens himself, Camp 020 made for an ominous first impression," states MI5's own website.