Documents from GCHQ, the Government's communication headquarters, confirmed the identity of two important Russian agents and reveals a third, hitherto unknown.
Some 57 files, all stamped "Top Secret", were released at the Public Record Office in Kew, detailed the impact on Britain of the traitors Guy Burgess, Donald McLean and Kim Philby, who passed thousands of documents relating to security to Moscow.
Also released were documents from a programme set up in the US in 1945 to check and decipher Soviet transmissions.
Codenamed Venona, it operated at Arlington Hall, Virginia. In 1948, Britain started working with the Americans and the joint programme was disbanded in 1980. The operation monitored the activities of atom spies like Klaus Fuchs, Theodore Hall and others whose activities meant Russia was able to explode its first nuclear device in September 1949.
The papers show that Fuchs was an active agent in 1941, at which time he was working in Birmingham under a Russian agent codenamed Sonia.
When the Cambridge-based physicist joined the top-secret Manhattan Project at Los Alamos, New Mexico, in the US to work on the atomic bomb, he was one of an elite team of scientists and Nobel prize winners led by Robert Oppenheimer. Also on the team was the British-based physicist Theodore Hall, a US citizen but also a Moscow agent.
He and Fuchs succeeded in passing almost every aspect of the bomb and its construction methods to their controllers.
Fuchs's codename was Charles, and four-and-a-half months before the first bombs levelled Hiroshima and Nagasaki in August 1945, Russia was praising his work.
A cable to New York from Moscow centre in April 1945 said: "Charles's information on the atomic bomb is of great value. Apart from the data on the atomic mass of the nuclear explosive and on the details of the explosive method of actuating BAL [Balloon, a codename], it contains information received for the first time from you about the electro-magnetic method of separation of Enormuz [the Atomic Energy Project].
"We wish in addition to establish the following: What kind of fusion - by means of fast or slow neutrons?"
Meanwhile, Hall, who was codenamed Mlad, had passed on details of the plants and manufacturing methods at the six factories building the bombs.
Fuchs was sentenced to 14 years imprisonment at the Old Bailey in March 1950 and was eventually was released in 1959 to go to East Germany.
Hall was never arrested and still lives in Cambridge with his wife Joan. After the war he did pioneering work on X-ray micro-analysis.
It is likely the security services decided not to prosecute in case it tipped the Russians off to how much of their cable traffic was being monitored. Hall's identity was revealed last December, by the Americans.
The papers make frequent references to Philby, Burgess and McLean, whose treachery is known to have cost the lives of hundreds of agents and almost severed relations between the US and British security services. The Americans were appalled at the number of breaches in security coming from Britain and openly expressed reluctance to become involved in joint operations.
McLean was working at the British embassy in Washington passing details of policy over the future of occupied territories back to his masters.
He stole cables at the highest security level. Because of the Russian espionage system's use of one-time code pads, just how much he had stolen was not finally discovered until July 1965.
Of the 2,200 signals released today, many are mere scraps, some with only a single word identified. But over time, the key senders were identified by their codenames. McLean was Homer, Burgess was Hicks, and Philby was Stanley.
In September 1945, Moscow centre was in a panic following the defection in Canada of Igor Guzenko, which prompted a major spy-hunt as he revealed secrets to the Canadians.
Moscow mounted a damage-limitation exercise in all areas, including Britain, instructing controllers to cut down on meetings and warning agents to take greater care. Philby, who held a senior post in British intelligence, passed back that the defection had caused disruption and led to the introduction of counter-measures. At the same time, Moscow sent a new agent, codenamed to Boris, to Britain to act as controller of a British spy known as Johnson.
General Pavel Fitin, head of Moscow centre, was worried whether the two would get on insisted that meetings with Philby and Burgess should not be increased in case they were spotted by intelligence-watchers.Reuse content