The KGB planted bugs to eavesdrop on John Profumo's pillow talk with Christine Keeler, according to newly released top-secret files.
The topless showgirl and model's KGB lover also persuaded her to question Profumo, Britain's Minister of War, about Britain's nuclear arsenal, the files reveal.
The reports claim that the Russians obtained 'a lot of information' which threatened to undermine Western security, contradicting the long-term view that the affair did not damage UK security and that no secrets were leaked to Russia.
And the files also reveal details of society dinner parties descending into debauched orgies, with, on one occasion, a naked Government Minister acting as a waiter.
Unknown to Profumo, his lover Keeler was also involved with the spy Yevgeni Ivanov, a naval attache at the Soviet embassy in London.
The affair's exposure in 1963 led to Profumo's resignation and rocked Prime Minister Harold Macmillan's Government.
The FBI files, declassified under the US Freedom of Information Act, reveal Keeler confessed Ivanov asked her to obtain information about the movement of Britain's nuclear warheads and that the operation led to Profumo being a 'blackmail victim'.
The files suggest the US, during delicate negotiations between Macmillan and President John F. Kennedy on the sale of the Polaris missile to the UK, feared defence secrets had been compromised.
The US took the affair so seriously, FBI director J. Edgar Hoover hand-delivered a note to US Attorney General Robert Kennedy, JFK's brother, warning of the risks.