The documents, including transcripts of secret wartime signals to London, are being released by the Public Records Office. They will show evidence of the role played by James Klugmann - the Soviet mole who converted the British spy, Donald Maclean, to Communism - in switching British allegiance from a Yugoslav royalist resistance leader called Mihailovich to Tito, at a critical point in the Second World War.
By switching support to Tito's forces, the Special Operations Executive (SOE) helped to force the German retreat, but it cost Mihailovich his life - he was executed after the war as a collaborator - and ensured that the former Yugoslavia remained a Communist state under Tito's control.
SOE spies who fought in the Balkans included the former Tory MP Julian Amery. Other famous names who flit in and out of the tales of SOE derring- do and duplicity in the region included Paddy Leigh Fermor and Major Anthony Quayle, the screen actor.
Rupert Allason, author of spy books under the pen name Nigel West, and a former Tory MP, said the real issue raised by the papers was the reason for the British Government's backing of Tito. Nothing had been known about Tito - Fitzroy Maclean, a British agent, thought he was a woman - and the Government became convinced that Mihailovich was a collaborator with the Germans - something the "Ultra" code intercepts showed to be untrue.
The signals sent by Klugmann, who was an intimate of the traitors Blunt, Philby and Burgess at Cambridge, will for the first time confirm the claim of an agent, quoted by Andrew Boyle in The Climate Of Treason, that Klugmann was principally responsible for the massive wartime sabotage of the Mihailovich supply operation and for keeping from London information about the impressive activities of the Mihailovich forces in the fight against the Germans.
They will be of particular interest to a decoder at Bletchley Park, nerve centre of the government's radio intelligence war, who, while preserving the anonymity of her wartime role, gave additional weight to the theory of Klugmann's secret agenda. "I was in section 3L at GCHQ Bletchley Park with the job of preparing a weekly summary of the Yugoslav situation for Churchill. At the time I wasn't particularly suspicious that our information didn't seem to be acted upon, but have become so since. I now wonder if many of our reports were sent to the section where people like Philby were working," she said.
"Certainly Klugmann seems to have played a more important role than was thought. Two former Communist wartime agents assured me that he did, but they didn't elaborate," she added.
The files, 969 in all, cover the operations of the SOE in Greece, Yugoslavia, Bulgaria and Romania; which, with the exception of France, was the most controversial theatre of the sabotage operation launched by Churchill "to set [occupied] Europe ablaze".
Unfortunately the blaze all too frequently singed SOE operatives themselves as they were caught up in internal politics - particularly in Greece and Yugoslavia. While fighting the German and Italian invaders, the Yugoslavs were simultaneously locked in combat with each other. Special Operations Executive (Balkans) operated from Cairo, and was ordered to carry out the policies of Churchill's government, which initially supported Mihailovich's royalist Chetnik forces.
The signals sent to SOE HQ in Baker Street, London, and to Churchill's Cabinet, were based in part on intelligence gleaned from German Ultra code traffic, filtered through Bletchley Park and passed to the only person in SOE Cairo authorised to receive it, Colonel C M (Bolo) Keble.
A further opportunity for slanting the information from Yugoslavia was provided by the influence exerted by John Cairncross, subsequently also unmasked as a Russian agent and named as the Fifth Man, recruited from the same Cambridge background, who in 1943,was working with the Yugoslav section of GCHQ at Bletchley Park.
The concerted efforts of the Cairo office eventually bore fruit when the British government dropped its support for Mihailovich. The Kew files are redolent of the suspicion and duplicity which blighted relations between SOE Cairo and its Foreign Office masters and which threatened to tear the intelligence community in the Balkans apart.
There is evidence of a power struggle which developed over the role of Brigadier Sir Fitzroy Maclean, who was parachuted in as Churchill's personal representative and came to exercise a powerful influence with Tito.
Two months later, Bill Deakin, later Colonel Sir William Deakin, Senior Intelligence Officer in Yugoslavia, rated Klugmann "indispensable ... and giving invaluable service." The file reveals that it was known that Klugmann had used his position to advance Tito's cause.