A British woman who passed along nuclear weapons secrets to Moscow for more than four decades was portrayed today as one of the most important spies ever recruited by the Soviet Union.

Melita Norwood, 87, a former secretary for a metals research group whose work was critical in developing the British nuclear deterrent, acknowledged today that under the code name Hola, she supplied invaluable information that probably enabled the Soviet Union to acquire the atomic bomb at least two years earlier than expected.

"I thought that perhaps some of what I had access to might be useful in helping Russia to keep abreast of Britain, America and Germany," the woman told reporters outside her home in a south London suburb.

The role played by Norwood, whose work was hailed by Russian spy masters as more vital than services performed by the infamous defector Kim Philby, is just one of many startling revelations said to be contained in a treasure trove of top-secret material smuggled out of KGB archives in Moscow in 1992.

The defection of KGB researcher Vasili Mitrokhin, who brought out six trunks of notes and copied documents recounting some of the KGB's most sensitive activities against Britain and the United States, was described by former Western and Russian spies--who gathered here this weekend for a conference on Cold War espionage--as one of the most extraordinary events in the intelligence game since the Soviet Union collapsed.

Christopher Andrew, a Cambridge University historian who is the author of a forthcoming book based on Mitrokhin's archives, said in an interview that the British Embassy in Latvia helped the KGB dissident to defect with his blockbuster material after CIA officials at the U.S. Embassy in Riga had rejected his pleas for assistance to flee his homeland.

"This turned out to be an intelligence coup of remarkable proportions because it fills in the missing pieces of so many puzzles," Andrew said.

As one of the top archivists at KGB headquarters, Mitrokhin reportedly kept notes for at least 12 years on major KGB operations against the West. He stashed them in his shoes or his pants as he left work and buried them in metal trunks under his house.

The archives are said to describe how the KGB tried unsuccessfully to recruit former secretary of state Cyrus Vance and former national security adviser Zbigniew Brzezinski.

The Mitrokhin papers are also said to show how the Soviets eavesdropped on White House and State Department communications, tapped the telephone lines of major American defense industries and planted spies in key companies whose information enabled Soviet engineers to build many advanced weapons systems according to pilfered American designs.

While many defectors on both sides of the Cold War were persuaded to betray their countries because of basic greed, some of the most important cases, such as Mitrokhin's decision to leave his country upon the breakup of the Soviet Union, seem rooted in a genuine disgust with their intelligence bosses or the directions of their homelands.

The history of Norwood's treason, which began in 1937, reflects the sympathy shown by many British leftists toward Soviet socialist ideals that persisted even after evidence that they were being perverted by the totalitarian excesses of Josef Stalin's regime.

Andrew said that well before her retirement in 1979, Norwood, or Hola, was awarded the Red Banner--the highest Soviet honor given any foreigner--and offered a lifetime pension. But she spurned all offers of financial reward and insisted that she was interested solely in supporting a classless society.

"In general, I do not agree with spying against one's country. My late husband did not agree with what I did," Norwood said today, in accounting for the motives that impelled her to photograph thousands of secret documents while she worked at the British Non-Ferrous Metals Research Association.

"I did what I did not to make money but to help prevent the defeat of a new system which had at great cost given ordinary people food and fares which they could afford, good education and a health service."

Oleg Kalugin, a former Soviet intelligence officer who now lives in the United States, confirmed the importance of the "Hola" case and suggested that other spectacular incidents may soon emerge from the Mitrokhin material.

CAPTION: Melita Norwood, 87, tells reporters at her home that she provided invaluable material to the Soviets.