The UK ambassador was astonished by President Richard Nixon's failure to destroy tapes that led to his downfall after Watergate, it has emerged.
Nixon could have saved his career, Sir Peter said
Nixon resigned in 1974 after the cover-up of a bungled break-in by a Republican Party "dirty tricks" team at the Democrat Party headquarters.
Ambassador to Washington Sir Peter Ramsbotham said Nixon would have survived if tapes had been destroyed.
He made his comments in a report, released to the National Archives.
Nixon's resignation was seen at the time as triumph for the constitutional system, a view Sir Peter was sceptical about.
In the lengthy report, made public under the 30-year-rule covering records of public office, he said Nixon's hand had only been forced after the release of the self-made incriminating tapes of White House conversations.
"The fact remains that had he decided to destroy the tapes as soon as, or before, knowledge of their existence became known he would almost certainly have escaped his fate," Sir Peter wrote.
That would have been "the wrong result" for the western world, he added.
He suggested that it may have been the desire of "persecution maniac" Nixon to book his place in history that sealed his fate.
"Perhaps the most likely reason is that, deep down, he wanted to preserve all the materials that would help him in retirement to write for himself his place in history," he said.
On Tuesday, a former deputy chief of the FBI was named as the man who became known as "Deep Throat", the source who leaked secrets during the scandal.
The Washington Post, which uncovered the scandal, confirmed that Mark Felt had owned up to being the source whose identity has been secret for decades.
Deep Throat helped Washington Post reporters Bob Woodward and Carl Bernstein uncover the Watergate affair.