The Queen was kept in the dark for years by a palace traitor, MI5 documents reveal

The decision to officially inform the Queen came amid growing concern in Whitehall that the truth would inevitably emerge after Blunt, who was seriously ill with cancer, died. Journalists were already investigating the story and were no longer constrained by concerns about defamation.

Suspicion first fell on Blunt in 1951, when fellow spies Guy Burgess and Donald Maclean fled to the Soviet Union.

He had been a close friend of Burgess since their time together in Cambridge in the 1930s – part of the so-called Cambridge Five spy group.

During the Second World War Blunt worked for MI5, after 1951 he was interviewed 11 times by the Security Service, but always denied espionage.

Then the American Michael Straight told the FBI that he was recruited as a Russian agent by Blunt himself.