THE BRITISH are fascinated with spies. Their hero used to be James Bond, the cool gentleman enforcer of impeccable good taste whose prowess and probity reflected the glories of old empire.

But now the British are concerned with the darker side of espionage, with tales of betrayal like the traitor at the top of the British Secret Service in John Le Carre's "Tinker, Tailor, Soldier, Spy." The new British interest is in the spy whose twisted or absent loyalties reflected Britain's own uncertainty as she declined from world power to being an island in a very large sea.

This interest is fed by Britian's history. In 1951 two diplomats, Guy Burgess and Donald MacLean, both of whom served in the British Embassy in Washington, defected to Russia. They had spied for the Russians for over 22 years and they mysteriously escaped just before being arrested by British security agents. The suspicion has always been that they were tipped off by another spy inside the British government.

After their escape, Kim Philby, the former head of the anti-Soviet section of British intelligence and also the British liaison with the FBI and CIA, also was suspected of being a Soviet spy. Philby resigned from British intelligence without having been conclusively proved a traitor. But a Soviet defector revealed in 1962 that he had been a KGB agent for over 30 years and a British intelligence officer subsequently confronted Philby, who confessed all. Remarkably, he too managed to escape to Russia in 1963.

There have been several other Soviet spies in the British government and Le Carre has based his best-selling spy novels partly on their exploits and what they reveal about British society. The BBC has just finished televising "Tinker, Tailor, Soldier, Spy" in a seven-week series that kept millions of Britons glued to their sets. It is clear they are fascinated with the questions of how and why some of their most important officials betrayed their country to the Soviet Union.

A new book is being published in Britain next week that tries to answer these questions. Called "The Climate of Treason," it was written by a former BBC news producer, Andrew Boyle, who has become one of Britain's most respected biographers. It is based on extensive interviews with former top CIA and British intelligence officers.

One of the major revelations in "The Climate of Treason" is that there was another important Soviet spy in the inner circle of British intelligence, one whose existence has not been publicized before. This man, code-named "Maurice," knew Philby, Burgess and MacLean from their student days.

The three men studied at Cambridge in the 1930's, when belief in communism was fasionable among the brightest students and homosexuality was prevalent. Boyle shows how in this atmosphere Burgess, Philby and MacLean formed their devotion to Russia and their links with one another.

Burgess was an ostentatious homosexual. Boyle claims Burgess seduced MacLean, who in later life would seek out homosexual company in fits of guilty drunkenness and extreme stress. Philby was not a known homosexual, but he was a close friend of Burgess for life.

"Maurice," Boyle says, was also a homosexual, a close friend of Burgess, a Communist and a teacher at Cambridge. During World World II, "Maurice" joined British intelligence, while spying for the Russians. It was he, Boyle says, who tipped off Burgess and MacLean to their impending arrest in 1951.

Because British libel laws are extremely strict, Boyle has not named "Maurice" in his book. But he is known to British security officials, because, Boyle claims, he confessed his treason to them after Burgess and MacLean escaped. In return for his confession and to avoid further scandal, "Maurice" was pardoned. Sources familiar with the case say he is a respected art historian, a former director of one of Britain's leading art institutes and had been knighted for his achievements before his role in the spy case was known.

Philby, Burgess, MacLean and "Maurice" all came from respected middle class backgrounds. They went to the best public schools and there acquired the proper identifying accent of the elite. MacLean's father had been a member of the Cabinet.

But each of these young men believed, for their own individual reasons, that the world of their fathers was doomed. Aided and urged on by other Communists, they began to work for Russia even as students. With World War II approaching, these young spies were ordered to enter the British government. With remarkable ease, they rose through the ranks.

None of these KGB agents was ever investigated before being accepted to the top echelons of the British government. Their known past allegiance to communism was passed over because they were "the right sort of chaps." Time and again, their connections preserved them against all the odds.

Inside the British government, rank amateurism and inefficiency allowed Philby and company to carry on. Boyle says Philby passed the names of most of the British agents in the world to Russia, as well as information given to the British by the CIA. He also estimates that Philby was responsible for the deaths of some three dozen spies whom he betrayed.

There is no reason to think that the other Britons who spied for Russia were any less destructive. MacLean is known to have passed important atomic secrets to Russia when he worked in the British Embassy in Washington.

Boyle says that on at least two occasions Burgess and MacLean each told friends of their treason -- but the friends either did not believe them or could not bring themselves to tell the authorities. This strongly British dedication to friendship reflects what Boyle calls "the terrible solidarity of the upper classes -- you never squeak on a friend." And Philby and company were masters of using this kind of dedication to preserve themselves.

But where the British failed to detect these Russian agents, Boyle remarkably asserts, the CIA succeeded. In the other major revelation of his book he claims that the CIA discovered a British nuclear scientist, code-named "Basil," spying for the Russians while working on Anglo-American nuclear research in Washington in 1948. The CIA, Boyle says, "turned" Basil and used him to uncover Philby, Burgess and MacLean.

The the CIA was so suspicious of British security that it did not tell the British of its discoveries, preferring, Boyle says, to wait and see what other spies might be revealed. To this day, Boyle says, British intelligence officials cannot quite understand or forgive the CIA for not telling them about the traitors who subsequently escaped to Russia.

The mad, ingrown, incestuous world Boyle has traced is now at least partly gone by. He and other observers say that British intelligence has been thoroughly reformed, with several "old boys" forced to retire and a new efficiency giving the service respect.

But it is clear that the seamy recent past of British intelligence still sticks in the public mind. When asked recently why this should be so, John Le Carre said: "Spies are at the front line of the loyalty business -- they serve where loyalty is most likely to be challenged and reflect the reality of what we are doing. Spies are statements of our national psyche."