Robert Kennedy intervened to suppress revelations about Jeremy Thorpe’s sexuality in order to save the British establishment from scandal, according to a previously unseen memo.

Kennedy was US Attorney General in 1963 when the FBI came into possession of a passionate letter from Thorpe to an American man known as “Bruno”.

Thorpe, then a rising star of the Liberal Party, was clearly besotted and wrote: “If I’m ever driven out of public life in Britain for a gay scandal then I shall settle in San Francisco,” the city where the two men had met.

It was a prescient remark, as in 1979 Thorpe was accused of conspiracy to murder his former lover in what was dubbed “the trial of the century”. He was acquitted, but his career was destroyed.

When the letter came to light in 1963, Britain was in the grip of the Profumo affair. At the time, homosexuality was illegal in Britain.

An FBI memo, obtained along with the letter by the BBC via a Freedom of Information request, reads: “The Attorney General said he learned of this letter during a visit to New York last week.