LONDON, OCT. 17 -- The Cold War may be over, but the Spy War goes on and on -- as does Britain's obsession with the identity of the so-called "Fifth Man," the senior British intelligence official who purportedly joined four other highly placed "moles" in betraying their country to the Soviet Union.

It's an old legend, one that has been immortalized in the fiction of John le Carre and Len Deighton and in Peter Wright's "Spycatcher." But its powerful hold over the British imagination and the controversy it still inflames are on display yet again this week with the publication here and in the United States of a new book by prominent KGB defector Oleg Gordievsky and British historian Christopher Andrew.

The book, which bills itself as "the first full, authoritative history" of the KGB, claims that Harry L. Hopkins, President Franklin D. Roosevelt's closest personal adviser, met regularly with a senior Soviet intelligence officer during World War II and served as an unwitting "agent of major significance" for the Soviets. "He was terribly naive, not terribly subversive," Gordievsky said at a news conference today.

The book also includes a detailed account of Operation Ryan, which Gordievsky says was a major KGB intelligence effort to counter what the Soviet leadership feared was a U.S. plan for a nuclear attack in the first term of the Reagan administration. It also alleges that Raoul Wallenberg, the Swedish diplomat who saved the lives of thousands of Hungarian Jews during World War II, was executed by the KGB after refusing to become a Soviet agent after the war.

But all of these claims have been overshadowed here by the Fifth Man issue. At today's news conference, the two authors were bitterly attacked by another prominent espionage writer, John Costello, who questioned their identification of retired British civil servant John Cairncross as the Fifth Man. Costello also accused the authors of writing a sanitized book approved in advance by Britain's spy chiefs yet deceptively marketed as "KGB: The Inside Story."

Andrew heatedly denied the accusations, and although he acknowledged the book had been reviewed by a government security panel, he denied it had stipulated any deletions or changes. Nonetheless, the dispute raised anew the question of why, nearly 40 years after the legendary Cambridge spy ring was first exposed and long after the deaths of most of the proven and purported participants, the question of the Fifth Man still haunts and intrigues Britain.

Gordievsky defected to the West in 1985 after serving as a British "penetration agent" for 11 years. A historian by training, he says that in 1980 he prepared a secret KGB history of its foreign operations, an assignment that allowed him access to thousands of documents he would otherwise have never seen, including material on the British spy ring.

Formed from disgruntled, upper-class scholars at Cambridge University in the 1930s, the ring is known to have consisted of Guy Burgess, Donald Maclean, Kim Philby and Anthony Blunt, all of whom went on to careers inside British intelligence. Their gradual exposure, the details of which the British government sought to conceal for years, devastated the intelligence establishment here and shook public trust in Britain's rulers.

For years, scholars, journalists and espionage buffs have argued over whether there was a Fifth Man and who it might have been. Inside the MI5 and MI6 spy agencies, a small group called the Young Turks even argued that the Fifth Man was Sir Roger Hollis, the former MI5 head who died in 1973.

Cairncross's involvement in Soviet espionage has been known for a decade, but Gordievsky and Andrew claim his role was much more broad and important than previously suggested. As a civil servant in the Foreign Office, private secretary to a wartime cabinet minister and member of the Bletchley Park code-breaking unit, Cairncross was in a unique position to pass on thousands of top-secret documents to Moscow, the book contends.

Cairncross, now 76 and retired in France, denied the charge in an interview with the Paris daily Le Monde. "I am not the Fifth Man," he said.

Some experts have remained skeptical of Gordievsky, suggesting that he could never have gained access to all the material he claims to have seen and retained. Some have said he simply has offered a version of the facts most likely to impress his spy handlers in Britain and the United States and to sell books, while others have even suggested he might be a "double double agent" -- still working for the KGB and spreading disinformation.

The book offers details on Operation Ryan, which Gordievsky describes as an attempt by the Soviets to monitor what they believed was a U.S. plot for a surprise nuclear attack. It was, said Andrew, the result of "a combination of Soviet paranoia and American rhetoric," an incident that illustrated the crucial difference between information-gathering and understanding.

It claims White House aide Hopkins met regularly with Iskhak Akhmerov, who it says served as Alger Hiss's Soviet controller during World War II. Hiss, a senior State Department official, denied charges he spied for the Soviet Union. The book says Akhmerov pretended to give Hopkins personal and confidential messages from Soviet leader Joseph Stalin during the war. It says Hopkins gradually became an ardent admirer of Stalin and helped persuade Roosevelt to aid the Soviet war effort in 1941 and to give the Soviets free rein in Eastern Europe as the war ended.

Gordievsky was perhaps the most senior KGB official ever to defect and British intelligence, eager to justify its work at a time of Cold War meltdown and defense budget cuts, has proudly paraded him in recent months. He has appeared twice on television documentaries, disguised by a fake hairpiece and goatee. He makes his American debut Thursday on ABC News's "Prime Time Live" and is scheduled to appear on NBC's "Today Show" Friday morning.