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The centenary of the founding of the security service MI5 will be marked today with the publication of the agency's first official history.
Cambridge historian Professor Christopher Andrew was given complete access to the services files, dating back to 1909 when it was set up to counter the threat from Russian spies.
It is thought to be the first time that any intelligence agency has opened up its archives in this way.
Extracts from The Defence Of The Realm already serialised in The Times have disclosed that MI5 kept a file on Harold Wilson throughout his years as an MP and Prime Minister.
But although the file was opened when Wilson first entered Parliament in 1945, and was maintained throughout his premierships of 1964-70 and 1974-6, Prof Andrew concluded that MI5 did not use the information to undermine him - despite his fears in his later years that they were plotting to oust him.
Among other episodes expected to be covered by the book are MI5's successful development of the Double Cross system during the Second World War to "play back" captured German agents to their Nazi spymasters, sending them false intelligence.
There is also detail on the unmasking of the infamous Cambridge Spy Ring - comprising Kim Philby, Anthony Blunt, Guy Burgess, Donald Maclean and John Cairncross. The Russian-recruited traitors are described as the ablest group of British agents ever to turn to a foreign intelligence service.
More recently, it may also shed fresh light on the MI5's controversial monitoring of domestic "subversives" - including trade unionists, anti-nuclear protesters, and civil liberties campaigners - during the 1970s and and 1980s.
It is also expected to look at the service's current focus on counter-terrorism, from the IRA and other bodies linked to Northern Ireland, to al Qaida-inspired militants.
