Christine Keeler, the woman at the centre of the Profumo scandal, claims she was made pregnant by the then Secretary of State for War.
She makes her claim in her autobiography, The Truth At Last, which is serialised in the Mail on Sunday newspaper.
Harold Macmillan's Conservative government was rocked to its foundations in 1963 when it emerged that call-girl Keeler had slept with John Profumo.
In her book she says that during the inquiry into the scandal Lord Denning ignored her evidence and then threatened her with prison in an attempt to cover up the truth.
During the scandal she was said to have also slept with Russian Naval Attache, Eugene Ivanov.
In the book, Keeler claims Roger Hollis, then director general of MI5, was the fifth member of the infamous Soviet spy ring which included Anthony Blunt, Kim Philby, Guy Burgess and Donald Maclean.
Several government inquiries into Hollis's role as MI5 chief have proved inconclusive.
Keeler claims the entire affair was orchestrated by her benefactor, Stephen Ward, a society osteopath and royal cartoonist, who ordered Ivanov to sleep with her.
Ward, she says, also ordered Ivanov to persuade her to start an affair with Profumo, who was widely seen as a possible successor to Macmillan as Prime Minister and someone with access to Britain's defence secrets.
Keeler claims Ward threatened to kill her when he thought she was about to expose him and says that she fell pregnant by Profumo but never told him and had a termination which nearly killed her.