LONDON (AP) _ The long-secret ''Fifth Man'' of Kim Philby's spy ring was John Cairncross, a former high-ranking British intelligence officer, according to a book by a former KGB double agent who fled to the West.

Oleg Gordievsky, the Soviet agent who came to Britain in 1985, makes the claim in a book, ''KGB: The Inside Story of its Foreign Operations from Lenin to Gorbachev.'' It was written with Christopher Andrew, a Cambridge University intelligence historian.

Serialization of extracts from the book began Saturday in London's Times newspaper. The book was to be released in Britain and the United States on Friday.

In Monday's extract, Gordievsky and Andrews say Cairncross was a spy before and during World War II. He worked during his spy years in the Foreign Office, the Treasury, for a government minister, for Britain's electronic eavesdropping center and for the MI6 foreign intelligence service, the book claims.

The Times said the Scottish-born Cairncross confessed to the MI5 counterintelligence service in 1964 after interrogation and that he lives in the south of France.

''This is not a new discovery,'' said Labor party legislator Ted Leadbitter. ''This has been known for an awfully long time.''

Harold ''Kim'' Philby, said to be the most damaging double agent in British history, spied for the Soviets for 26 years until he defected to Moscow in 1963. He and three fellow agents, Guy Burgess, Donald MacLean and Anthony Blunt, all now dead, turned Britain's secrets over to the Soviets.

Some scholars of espionage have long suspected that Soviet penetration of the British secret service did not end with the unmasking of the four and that there was probably a ''Fifth Man.'' Others have maintained that the KGB fostered the theory to keep Western intelligence in doubt.

It was at Cambridge University, where all five of the men studied, that Blunt ''talent-spotted'' Cairncross and Burgess recruited him, the excerpt says.

Philby, who rose to become head of the MI6 anti-Soviet section, responsible for the study of communism and the Soviet intelligence services, was on his way to becoming head of MI6 when he first came under suspicion in 1951, when Maclean and Burgess fled to Moscow.

The Times said Cairncross's involvement with Soviet intelligence has been well documented, but he had never been placed in the same category as the other four. ''Cairncross always claimed he was just 'a small fish'.'' the Times said.

On the contrary, ''Cairncross's achievements were the equal of any of the five except Philby,'' the excerpt quotes KGB officer Dmitri Svetanko as saying.

''Until the end of World War II, when Philby's career began to take off, I think he provided more important intelligence than any of the others,'' co- author Andrews told The Associated Press.

Writer Chapman Pincher and former MI5 officer Peter Wright, whose book ''Spycatcher'' the British government failed to suppress in a long court battle, both alleged that former MI5 head Sir Roger Hollis was a Soviet spy.

Earlier this year Gordievsky said in a BBC television interview that Hollis, who ran the counterespionage service from 1956 to 1965, was not a Soviet agent.

Gordievsky began working for the British while posted in Copenhagen in the early 1970s.

The KGB Soviet intelligence service sent him to London but he was recalled to Moscow. He eluded his KGB tail and escaped to the West in 1985. His defection led to the retaliatory expulsion of 31 Soviet and 31 British diplomats, trade representatives and journalists.