When Soviet spies Guy Burgess and Donald Maclean fled to Russia in 1951, their comrade Anthony Blunt refused to follow. Blunt’s handler, Yuri Modin, had assured him that he would be given a warm welcome in Moscow, but the Englishman dismissed the idea with a sneer: ‘No doubt you can also guarantee me total access to the Palace of Versailles?’

Blunt had no intention of slumming it behind the iron curtain.

 Although he had been attracted to communism as an idealistic Cambridge undergraduate in the 1930s, these days he was thoroughly disillusioned by Stalin’s atrocities. In any case, Blunt was an art historian with an international profile.

The inscrutable Englishman, right, managed to keep his cover until 1979, when Mrs Thatcher announced to Parliament that he was ‘the fourth man’ of the infamous Cambridge spy ring. 

The public unmasking caused a huge scandal, not least because, as Surveyor of the Queen’s Pictures, Blunt had on many occasions been alone with Her Majesty.

In this thrilling book, as twisty as a Le Carre, Piers Blofeld takes advantage of the recent declassification of a rich haul of MI5 papers to explore how Anthony Blunt’s treason was covered up for so long. Even though Blunt had confessed to MI5 as early as 1964, the secret was kept from the Queen for another decade.

Remarkably, when she was finally told, Her Majesty did not seem outraged. She had always felt grateful to Blunt for clearing the way to her marriage to Prince Philip, with whom she had fallen in love as a teenager. 

In 1945 Blunt had been sent to Germany by MI5 to ‘tidy up’ the embarrassing fact that three of Philip’s sisters were married to leading Nazis.

Four years into her reign, Blunt received a knighthood.

More extraordinary, Blofeld thinks that he also passed on information to the Nazis.

 New evidence suggests that in 1944, using the codename ‘Josephine’, Blunt leaked details of Allied military plans, with the result that thousands of British servicemen lost their lives. ‘What Blunt did was monstrous,’ says Blofeld, ‘his actions are unforgivable.’

Piers Blofeld – his surname was nicked by Ian Fleming from the author’s grandfather and given to the arch Bond villain – has written the fullest account so far of Anthony Blunt’s treachery. 

But still, of course, the central mystery remains: what made this quintessential Englishman, who was happiest in an art gallery, decide to betray his country?