Â"There are thousands (of agents identified) but of course they extend around the world,Â" said Christopher Andrew, a Cambridge academic and co-author of The Mitrokhin Archive: the KGB in Europe and the West.
Â"Absolutely nobody who spied for the Soviet Union in any part of the world between the Bolshevik revolution in 1917 and the mid-1980s can be certain that his or her secrets are safe.Â"
The book, which is being serialized in the Times of London newspaper, constitutes Britain's biggest security lapse for decades. And while the evidence, which was gathered by a lone KGB dissident, thus far has centered on Britain, Andrew made it clear the implications would reverberate worldwide.
Â"The FBI says that it's the single most important and comprehensive intelligence source ever obtained by any individual,Â" Andrew said in an interview.
The dissident, Vasili Mitrokhin, had access to all the files in KGB foreign intelligence and squirreled away data for 12 years before smuggling it out of Moscow in 1992, he said.
Â"That has current as well as historical consequences because the only way that you keep the loyalty of your current agents and the only way you can make a pitch for new ones is if people believe that you can keep their secrets,Â" said Andrew.
He said the number of spies to be unmasked in Britain was Â"well into double figuresÂ" and numbered two former Labor members of parliament, Tom Driberg and Raymond Fletcher, both now dead.
Driberg, a homosexual, was recruited in 1956 in a public toilet in Moscow, he said, showing that truth in the world of spies is often stranger than fiction.
Parts of the scandal read like a second-rate spy novel, with agents trained in lovemaking by Russian beauties and booby-trapped sabotage kits hidden in Swiss fields.
Â"Home Secretary Jack Straw has asked ... for a report on these revelations,Â" a spokesman for the Home Office (interior ministry) said.
Prime Minister Tony Blair's office said Straw would make a statement Monday. It said Blair knew nothing of the scandal until the revelations appeared in the Times.
Britain has not seen such a spy scandal since the unmasking of a Cambridge spy ring, dubbed the Magnificent Five by the KGB, a generation ago.
So far the revelations have focused on two agents: a crooked policeman and an ideological great grandmother.
John Symonds -- a former officer in Scotland Yard's pornography squad -- was alleged to have worked for the KGB for more than 10 years and slept with employees of British embassies to extract secrets.
Â"I was taught how to be a bettelover,Â" Symonds told BBC television. Â"I was taught by two extremely beautiful girls.Â"
His unmasking followed that of 87-year-old Melita Norwood, who passed atomic secrets to Moscow for more than 40 years, helping to give Moscow a vital edge in the arms race.
Norwood told reporters Saturday that she acted on principle.
Â"I did what I did not to make money but to help prevent defeat of a new system whichÂ… had, at great cost given ordinary people food and fares which they could afford, good education and a health service,Â" she said.
CBS News Senior European Corresondent Tom Fenton adds that there were no immediate plans to put Norwood on trial for espionage because of her age.
©1999 CBS Worldwide Inc. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed. Reuters contributed to this report