When the Nixon administration was in court a decade ago in an effort to cut off publication of the Pentagon Papers, the White House was told that a complete set of the top-secret documents had been delivered to the Soviet Embassy.

The FBI and the Nixon White House were evidently convinced that the report was accurate. It had come from "Fedora," a strategically placed KGB officer whom the FBI had been relying upon for years as a trusted counterspy.

The information he provided in this instance helped prompt formation of the infamous "White House plumbers" unit whose operatives later carried out the Watergate break-in. President Nixon's efforts to curtail the Watergate investigations were said to have been motivated in part by fears that "Fedora" would be exposed.

It might have been better if he had been. The FBI is now convinced that "Fedora" was a Soviet agent, acting under Moscow's control during all the years he fed information to the bureau.

The startling new assessment of "Fedora," until now a closely held secret, is disclosed in a forthcoming article in the October Reader's Digest and has been confirmed independently by The Washington Post.

The secret conclusion was based "to some degree, on new information," said one official familiar with the FBI's counterespionage effort. "It's an incredible business . . . an incredible chess game that you have to play."

The new finding about Fedora, who was stationed at the United Nations as a Soviet diplomat, also raises unsettling questions about the credentials of other supposed Soviet defectors, especially those whose stories Fedora backed up.

"If one falls, others must fall," contends the Digest article by roving editor Henry Hurt, "creating havoc inside intelligence services where crucial analyses and long-term plans may have been built upon the supposed reliability of these sources."

The case has all sorts of permutations. Much of the fallout concerns Yuri Ivanovich Nosenko, a onetime KGB officer who has been a bone of contention since he defected to this country in 1964 with claims that he had been in charge of the KGB file on President Kennedy's assassin, Lee Harvey Oswald.

Nosenko, who first offered to spy for the United States in 1962, had once said he would never defect, but then told his CIA contacts in Geneva in 1964 that he had to defect at once because he had received a cable from Moscow recalling him. He said he was afraid the KGB had learned of his contacts with the CIA.

From his post within the Soviet apparatus at the U.N., Fedora offered confirmation, telling the FBI that Nosenko had indeed been sent a recall telegram.

Nosenko also claimed to have been a lieutenant colonel in the KGB. Fedora, who had been providing information to the FBI since 1962, confirmed that claim, too.

Subsequently, however, Nosenko acknowledged under hostile questioning by CIA officers that his talk of a recall telegram had been a lie and so was his claim of colonel's rank. He had been only a KGB captain and had lied, he said, to exaggerate his importance.

Before long, Nosenko found himself imprisoned by the CIA for some five years, three of them in solitary confinement, but he never broke down and was finally rehabilitated in 1968. He became a consultant for the agency, collecting some $500,000 over the next decade in consultant salaries, bonuses, resettlement expenses and other payments.

Fedora, by contrast, appears to have had nothing but smooth sailing with the FBI despite his corroboration of Nosenko's admitted lies.

"When we started up with Fedora, the bureau held very strong views that he was legit," one former intelligence official recalled yesterday. "Of course there was a minority that felt the other way, but not many."

Much of what Fedora said over the years was, in turn, conveyed directly to the White House, enhancing his position, sources said. "There is no question the information would always go to the highest levels," said one expert. "That gave it a great deal of prominence."

By the time of the Pentagon Papers incident in 1971, editor Hurt said in a telephone interview, Fedora was "regarded as a knight in shining armor. And he was telling the FBI which was telling Nixon that a copy of the Pentagon Papers had been delivered to the Soviet Embassy. Nixon & Co. accepted the report without question."

In fact, word of the alleged delivery was quickly published by a conservative columnist with close ties to the White House while the Pentagon Papers case was still before the Supreme Court. Some critics regarded it as a White House effort to influence the court's decision, albeit an unsuccessful one.

In any case, according to a Dec. 9, 1973, New York Times article, President Nixon developed fears, reportedly nourished by his then-national security adviser Henry A. Kissinger, that Daniel Ellsberg, the man who had leaked the Pentagon Papers to the press, might have provided the Soviets with far more important secrets, especially concerning nuclear targeting plans.

Some intelligence officials were reportedly stunned that Fedora's word should be so readily accepted, without any further evidence. Skeptics such as CIA counterintelligence chief James J. Angleton had long regarded the Russian as an agent provocateur. But the White House wasn't listening.

"This could be a classic case of an agent sowing disruption at the highest levels of government," Hurt suggested in a telephone interview.

Fresh doubts were finally stirred in 1978, primarily about Nosenko but also about Fedora, with the publication of a book by Edward Jay Epstein called "Legend: The Secret World of Lee Harvey Oswald." It questioned the loyalty of both Russians. Subsequent investigation by the House Assassinations Committee showed that Nosenko had also lied about Lee Harvey Oswald and made other claims concerning the Kennedy assassination that even CIA officials found "incredible."

Nosenko had insisted, for instance, that the KGB had never even interviewed Oswald during his stay in the Soviet Union, much less recruited him as an agent. He also once denied any KGB physical and technical surveillance of Oswald in Russia, but later conceded that Oswald's file was crammed with surveillance reports.

The CIA, however, stuck by Nosenko in an unusual Sept. 21, 1978, public statement calling him "a well-adjusted American citizen utilized as a consultant by CIA and . . . making a valuable contribution to our mission."

But the FBI, meanwhile, undertook a fresh assessment of Fedora. He had reportedly returned by then to the Soviet Union, but files are kept in such cases of all the information supplied by such spies, including details on what proved to be true and what proved not to be true.

"Whenever you have an individual who claims to be a defector, you always have a question about his bona fides -- always," said one official familiar with the process. "You can say for 15 years that someone's great, but that doesn't take care of the 16th year. You've got to be constantly evaluating. For instance, if you have a defector in 1975 and one in 1970, you ask the one who comes in in l975 about what was happening in 1970."

The FBI's secret conclusion, reached in 1980, was that Fedora had been loyal to the KGB all along, including, Hurt emphasized, "the period when he was giving urgent support to Nosenko." But the intelligence community, Hurt said, has yet to undertake a re-examination of such related cases and sources.

Across the river, at Langley, the CIA had nothing new to say.

"It is our policy not to make public comment on such intelligence matters," said CIA spokesman Dale Peterson. "CIA's statement on Nosenko in 1978, however, stands."