MI5 papers reveal the Queen has been kept in the dark about a palace traitor for years

Queen Elizabeth II was not officially informed for almost a decade that one of her senior courtiers had confessed to being a Soviet spy, according to newly released MI5 files.

Art historian Anthony Blunt was for decades the Queen's portrait surveyor, overseeing the official royal art collection, and in 1964 admitted that he had been a Soviet agent since the 1930s.

Papers published by MI5 show that although Blunt admitted to them that he had spied for the Russians during World War II, the late Queen herself was not officially informed for almost nine years.

When she was told the full story in the 1970s, she was characteristically calm, dealing with everything “very calmly and without surprise,” according to declassified files released to the National Archives.

The decision to formally inform the Queen came amid growing concerns in Whitehall that the truth would inevitably emerge after the death of Blunt, who was seriously ill with cancer. Journalists were actually investigating the story and were no longer constrained by libel concerns.

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

He was a close friend of Burgess from their time together in Cambridge in the 1930s, and they were part of the so-called Cambridge Five spies.

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

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

In April 1964, he confronted MI5 investigator Arthur Martin Blunt, promising him immunity from prosecution.

His full confessions are included for the first time in these files. In addition to acknowledging his wartime work, he admitted that he had contact with the Russian intelligence service after the war.

Blunt said he met a Russian named Peter before Burgess and Maclean left, but he did not remember exactly why. He said that the person called Boutros encouraged him to flee as well, but he refused.

The investigator said that Plant was “uncomfortable” while speaking, and each question was “followed by a long silence” while he “appeared to be discussing with himself how to answer it.”

Despite Blunt's high profile, few outside MI5 were told of this confession. The Minister of Interior and his senior government officials have been informed.

Only the Queen's private secretary was told that Blunt was involved and that MI5 intended to question him.

It was agreed that if Blunt became seriously ill, she would be formally informed, as this might prompt press coverage of his past.

In March 1973, another file note indicates that the Queen's private secretary spoke to her about the Blunt case. It read: “I treated the matter very calmly and without surprise: I remembered that it had been in doubt in the wake of the Burgess/MacLean affair.”

Miranda Carter, Blunt's biographer, said her “hunch” was that Elizabeth II had been informally informed sometime after 1965.

She believes officials “wanted to maintain a veil of plausible deniability.” The Queen's acceptance of the news “calmly and without surprise” suggests to Carter that she must have known.

Prime Minister Margaret Thatcher finally revealed Blunt's past in a statement to the House of Commons in 1979. He died in 1983 at the age of 75 after being stripped of his knighthood.

• Cambridge spy Kim Philby announced he would do it again after finally admitting he had been a Russian agent for years

• Blunt feared his KGB handler would turn violent when he refused to join fellow spies Burgess and Maclean and flee to Russia.

• Film star Dirk Bogarde has been warned by MI5 that he could be the target of an attempted gay 'trap' by the KGB.

• MI5's chief investigator Philby was puzzled when he admitted he could not determine whether he was a Soviet spy

Unlike government departments, MI5 is not subject to the Freedom of Information Act. He edits his archives as he chooses and some files are partially redacted.

Some of the documents released today will be displayed in an upcoming exhibition at the National Archives.

MI5 Director General Sir Ken McCallum said: “Although much of our work must remain secret, this exhibition reflects our ongoing commitment to being open where we can.”