Queen Elizabeth II was not formally informed for nearly a decade that one of her most senior courtiers had confessed to being a Soviet spy, according to newly released MI5 documents.

Art historian Anthony Blunt, who for decades served as surveyor of the Queen’s paintings and oversaw the official Royal Art Collection, admitted in 1964 that he had been a Soviet agent since the 1930s.

Documents released by MI5 show that despite Blunt admitting to them that he spied for the Russians during World War II, the late Queen herself was not formally notified for almost nine years.

When she learned the full story in the 1970s, she acted unflappable, “very calm and not surprised,” according to declassified documents released by the National Archives.

The decision to formally notify the Queen comes amid growing concerns in Whitehall that the truth will inevitably emerge following the death of Blunt, who was suffering from serious cancer. Journalists are already investigating the story, and they no longer have to worry about libel.

Blunt first came under suspicion in 1951 when fellow spies Guy Burgess and Donald Maclean fled to the Soviet Union.

He had been a close friend of Burgess’s since their time together in Cambridge in the 1930s, and was part of the so-called “Cambridge Five” spy ring.

During World War II, Blunt worked for MI5. After 1951, he was interviewed by the security services 11 times, but always denied espionage.

American Michael Street then told the FBI that he had been recruited as a Russian agent by Blunt himself.

In April 1964, MI5 interrogator Arthur Martin confronted Blunt and promised him immunity from prosecution.

The files contain his full confession for the first time. In addition to acknowledging his wartime work, he also acknowledged maintaining contacts with Russian intelligence after the war.

Blount said he met a Russian named Peter before Burgess and McLean left, but he couldn’t remember why. He said the so-called Peter also encouraged him to escape, but he refused.

The interrogator said Blunt was not “relaxed” when he spoke and that each question was “followed by a long pause” and he “seemed to be debating with himself how to answer.”

Despite Blunt’s prominence, few outside MI5 were aware of his confession. The Home Secretary and his most senior civil servants were told.

The Queen’s private secretary was told only that Blunt was implicated and that MI5 planned to question him.

It was agreed that if Blunt became seriously ill, she would be formally notified as that could prompt media coverage of his past.

In March 1973, another archival record revealed that the Queen’s private secretary had discussed the Blunt case with her. It read: “She took it all very calmly and was not surprised: she remembered that he had been under suspicion long after the Burgess/McLean case”.

Blunt biographer Miranda Carter said her “hunch” was that Queen Elizabeth II was informed informally sometime after 1965.

She believed officials “wanted to maintain a plausible deniability attitude.” The Queen accepted the news “calmly and without surprise” and Carter thought she must have known.

In 1979, British Prime Minister Margaret Thatcher finally revealed Blunt’s past in a House of Commons statement. He died in 1983 at the age of 75 and was stripped of his title.

• Cambridge spy Kim Philby finally admitted that he had been a Russian agent for years – and he declared he would do it again.

• Blunt feared his KGB superiors would become violent when he refused to flee to Russia with fellow spies Burgess and MacLean

• MI5 warns film star Dirk Bogarde that he could be targeted for gay ‘entrapment’ by KGB

• MI5’s top interrogator was baffled by Philby, admitting he couldn’t determine whether he was a Soviet spy

Unlike government departments, MI5 is not subject to the Freedom of Information Act. It publishes its archives according to its own choice and some files are partially redacted.

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

Sir Ken McCullum, director-general of MI5, said: “While much of our work must remain secret, this exhibition reflects our ongoing commitment to being as open as possible.”