QUEEN ELIZABETH II was left in the dark for almost a decade over the full scale of the actions of one of her most senior courtiers, according to newly released official files.

In 1964, Sir Anthony Blunt, the surveyor of the Queen’s pictures and distinguished art historian, finally confessed he had been a Soviet agent since the 1930s.

He was recruited, when he was a young don at Cambridge, into one of the most notorious spy rings of the 20th century, the “Cambridge Five.”

As a senior MI5 officer during the second world war, Blunt had passed vast quantities of secret intelligence to his KGB handlers.

But he was nevertheless allowed to maintain his position at the heart of the British Establishment amid fears of a major scandal if he was sacked and the truth became public.

When the Queen was finally told the full story in the 1970s, she was characteristically unflappable.

She was observed to have taken it “all very calmly and without surprise,” according to declassified MI5 files released to the National Archives in Kew, west London.

The decision to ensure she was properly informed came amid growing concern in Whitehall that the truth would inevitably come out when Blunt, who had been seriously ill with cancer, died and journalists were no longer restrained by concerns of libel.

In February 1973, then prime minister Edward Heath ordered preparations to be put in place for dealing with the expected torrent of negative publicity.

These included instructions to Sir Martin Charteris, the Queen’s private secretary, to inform the monarch.

On March 19, MI5 director general Michael Hanley reported that the cabinet secretary Sir Burke Trend had shown him a “personal manuscript letter” from Sir Martin confirming she had finally been told.

The Queen was said to have been “not at all keen on Blunt and saw him rarely.”