Sir Anthony Blunt, one of the world's most distinguished art historians and the retired personal art curator to the queen, was today unmasked as a self-confessed Soviet spy here during World War II and immediately stripped of his knighthood.

In an unprecedented action, Prime Minister Margaret Thatcher ended weeks of growing speculation that had gripped Britain's establishment by issuing a statement in Parliament detailing the 72-year-old Blunt's involvement in the notorious wartime Burgess-Maclean spy ring here.

Thatcher's surprise statement, in answer to formal questions on the case from two opposition Labor members of Parliament, stunned the House of Commons and dominated news reports on a day when a major agreement was reached in the Rhodesian peace conference here and Britain's minimum lending rate was raised to a record 17 percent.

As Chancellor of the Exchequer Goeffrey Howe defended the government's decision to raise interest rates so high, Labor members excitedly passed copies of Thatcher's statement along rows of benches in the oak-paneled chamber.

The statement detailed how Blunt acted as a talent-spotter for the Soviet secret service while teaching at Cambridge University here before World War II, "passed information regularly to the Russians" while working for British intelligence during the war, and helped British diplomats Guy Burgess and Donald Maclean escape to the Soviet Union in 1951 when they were about to be arrested as Soviet spies.

He confessed his crimes secretly to the British only after being given immunity from prosecution in 1964, according to Thatcher, and was then allowed both to keep his knighthood and continue as Queen Elizabeth II's personal adviser on her art collection, reputedly the largest in the world.

Today, however, immediately after Thatcher's statement, Buckingham Palace announced that Blunt would now be stripped of his knighthood, the Royal Victorian Order, a distinction awarded only for personal service to the royal family.

The order stripping him of his knighthood, signed by the queen, becomes effective Friday, depriving Blunt of the honor of being referred to as Sir Anthony, a change already made in Thatcher's statement today, Blunt becomes the first person to lose his knighthood since World War I and the first member of the Royal Victorian Order to be so punished.

When asked why this was being done today rather than 15 years ago when the queen first learned of Blunt's treason, a palace spokesman said, "because now his activities have become public knowledge. Before it was private."

It might never have become public if it were not for British author Andrew Boyle, whose recently publishedbook on the Burgess-Maclean case, "The Climate of Treason," revealed that there was a fourth spy in the ring (the third was British Soviet counter-intelligence chief Kim Philby).

Strict British libel laws prevented Boyle from naming Blunt, but his booked was strewn with clues that led to speculation in drawing rooms and the more daring parts of the British press.

Blunt's name was first linked publicly to the Burgess-Maclean case earlier this week in the written parliamentary question to Thatcher. Everything said in Parliament is protected from libel action (Philby was first named as a Russian spy in House of Commons debate.)

"I didn't name Blunt in my book," Boyle said in an interview tonight. "It wasn't my job. It's for the government to say these things. I'm not in the business of doing its dirty work.

"I'm a Christian and I think this is fair," he added when asked his feelings about Blunt. "If he has a conscience, I'm sure it will weigh heavily tonight."

Blunt, a tall distinguished-looking man with perfect manners, was considered one of the most brilliant young men of his generation. In 1932, at the age of 25, he was made a fellow of Trinity College at Cambridge, where he was a prominent academic Communist and a close companion of Guy Burgess.

Blunt served in British counter-intelligence World War II and was reported to have been sent into Germany to rescue the private correspondence of Queen Victoria with her German relatives from the advancing American army. In 1945, Blunt was made curator of the royal art collection for King George VI and then for Queen Elizabeth II. A Buckingham Palace spokesmen said today that Blunt continued to advise the queen on her paintings until his retirement last December.

Besides being knighted in 1956, Blunt was the acknowledged expert on the work of the 17th Century French painter Poussin. He wrote several books on art and held enough distinguished academic positions for a three-inch listing in "Who's Who."

He has been a professor of fine art at Oxford University, Cambridge University and the Unversity of London, holds several honorary degrees, and was the director of London's prestigius Courtauld Institute of Art from 1947 to 1974.

People close to Blunt reacted with shock to today's disclosure. His brother, 75-year-old retired banker Christopher Blunt, said, "I never suspected anything." A spokesman for Blunt's London club said only, "I am sure any member would be most reluctant to talk about someone mixed up in anything like this."

Blunt himself could not be reached tonight. He was reported by his apartment building's porter to have left home at 7 a.m. today with several suitcases.

When asked about Mrs. Thatcher's statement today, author Boyle said, "I give her credit for making a clean breast of things -- she cut through all the old waffle that we had from past prime ministers like [Harold] Macmillan and [Douglas] Home." They had denied that there were any other Soviet spies in the British government.

Boyle said some members of the British establishment have long known of Blunts' treason. While he was working on his book, a Conservative member of Parliament told Boyle's son, according to the author, "I say, your father is going to give our Blunty a bit of a rough ride isn't he?"