Foreign and Commonwealth Office (FCO) historians have had access to intelligence documents and official papers to write the history of East- West relations at the height of the Cold War. It is the first time since the Second World War that a government has allowed the publication of any inside information in advance of the rule which blocks the release of papers for at least 30 years.
Among the files are details of how, in 1971, the new Tory administration under Edward Heath decided to get tough over the scale of Soviet intelligence activity in Britain, and an attack on Henry Kissinger, the United States Secretary of State.
Many of the files will remain unavailable to the public. But where the information is no longer regarded as a threat to intelligence operations or to international relations, correspondence and memoranda will be released verbatim - in two volumes, with a historical context provided by the FCO historians.
A previously confidential memorandum to the then Prime Minister, Edward Heath, from Sir Alec Douglas-Home, then Foreign Secretary, and Reginald Maudling, then Home Secretary, in July 1971, laid out in detail the problem of Soviet spying in Britain. They concluded that there were at least 120 intelligence agents in Britain and the total could exceed 200.
The various Soviet establishments, including the embassy, the trade delegation and the Moscow Narodny Bank, employed more than 500 people between them, making the Soviet delegation to Britain the largest of any country in Western Europe. The memo concluded that if the Soviet delegation to the United Nations was excluded, there were more Russians in Britain than even in the United States.
The volumes also detail the Soviet reaction, much less extreme than had been feared. There was, though, a freezing of relations and the Moscow establishment made life very difficult and confusing for the new British ambassador in Moscow, Sir John Killick.
In an appeal to the FCO for information to help him in his new post, he was left intrigued by the workings of Soviet intelligence. Apparently baffled, he asked "whether the KGB, for all their resources and efficiency, are out of their minds?"
The papers are expected to give details of the revelations of Oleg Lyalin, a KGB defector to Britain in 1971, who admitted he was responsible for plans to sabotage a radar station in North Yorkshire, built to give early warning of a Soviet nuclear missile attack.
Other areas likely to be covered include Gerald Brooke, a businessman falsely held on spying charges in Moscow and eventually released in exchange for the Krogers - Jewish-Americans whose real names were Morris and Lona Cohen. The Krogers were originally involved in the Rosenberg spy ring which was responsible for stealing atomic bomb secrets from the United States in the early Fifties.
The papers will also make it clear that Britain remained highly suspicious of the alleged US detente with the Soviet Union. They reported that Dr Kissinger had come to see the ground-breaking Conference on Security and Co-operation in Europe in Helsinki as a "positive obstacle to his task of developing Soviet-American co-operation".
They feared he was influenced by the Watergate affair and willing to stop putting the Soviets under pressure on human rights in order to achieve the US's own ends. Diplomats noted of Kissinger: "He often gives the impression that detente is primarily a matter of inter- governmental accommodation and that the human aspect is secondary."
The volumes of papers are to be launched at a special "witness seminar" which is expected to be attended by key international figures of the era. Lord Callaghan, the former prime minister; Lord Healey, former defence secretary; senior Foreign Office diplomats; and one-time heads of the intelligence services are among those invited to what is being regarded as an unprecedented gathering. Yuri Fokine, the Russian ambassador who was personal assistant to Andrei Gromyko as Soviet foreign secretary, has already indicated his enthusiasm to take part.