The Senate Intelligence Committee has begun an inquiry into hotly disputed assertions that the United States was duped by a phony Soviet defector who came to this country in 1964.

According to a new book, two branches of the Central Intelligence Agency concluded that the defector was probably a Soviet plant, but their conclusion was overrriden and the defector is now actively employed as a consultant to the CIA.

An independent inquiry by The Washington Post has established that the book's account is essentially correct, and that doubts about this defector sharply divided the U.S. intelligence community. The doubts are dismissed as unfounded by the FBI and other CIA officials, including former director William E. Colby.

The Post has also found that some former high-ranking CIA officials believe that acceptance of the defector as legistimate has gravely compromised some U.S. intelligence programs, perhaps even rendering them useless in the secret cold war with the Soviet Union.

The defector in question is Yuri Nosenko, who defected to the United States in February 1964, and claimed to have intimate personal knowledge of Lee Harvey Oswald's two-year stay in the Soviet Union before the assassination of President Kennedy. The defector said Soviet officials regarded Oswald as suspicious and had no substantive dealing with him.

CIA suspicions about Nosenko led to his being held virtually a prisoner for about three years and subjected to intense questioning. The Rockefeller Commission that reported on CIa acitivities in 1976 described Nosenko's handling without naming him:

"For much of this time [three years] the defector was held in solitary confinement under extremely spartan living conditions. The defector was apparently not physically abused. The justification given by the CIA for the lengthy confinement arose out of a substantial concern regarding the defector's bona fides . . ."

According to author Edward Jay Epstein, whose new book "Legend" was published April 9, senior officals in the CIA concluded that Nosenko was sent to the United States with a reassuring message about Oswald by the Soviet Committee on State Security, the KGB, Epstein charges that after exhaustibe investigation the CIA and other U.S. agencies decided that they could not decide whether Nosenko was legitimate or phony.

Later, Epstein contends, new executives in the CIA cleared Nosenko and declared him bonafide.

The staff of the Intelligence Committee is now investigating these events, according to a member of the committee.

The Post's inquiry into Epstein's allegations was hampered by the fact that although sources who sympathized with his viewpoint were not available for questioning.

One former CIA official who was a source for Epstein's book told The Post he had read the final CIA report on Nosenko that was the basis for clearing him. This former official claimed that the report did not respond to dozens of the questions raised about Nozenko's reliabiltiy, but merely concluded that he was a self-serving liar, not a planted KGB agent.

The CIA refused to discuss the case, except to say: "We are satisfied with Nosenko's bona fides."

The basic challenge to Nosenko's reliabiltiy came from the CIA's counterintelligence division, then led by James J. Angleton, and the Soviet Russia Division, then led by David Murphy and Tennent H. Bagley. Angleton-who is well known for his suspicious view of defectors in generl and his great respect for KGB wiliness-was fired from the CIA by Colby. Murphy and Bagley apparently both resigned.

The Post's inquiry has established that senior U.S. intelligence officials, including at least some of those who were fired or retired in various personnel upheavals at the agency, fear that the ultimate acceptance of Nosenko has effectively destroyued the ability of U.S. intelligence organs.

Some of these officials harbor fears that the Soviet Union has effectively penetrated the CIA, the FBI or both.

At the same time, The Post has determined, other intelligence officials, including those now in charge of U.S. intelligence activities, dismiss these dark views as unwarranted and perhaps paranoid.

Epstein and informed sources sympathetic to his viewpoint contend that the Nosenko case is particularly significant because it is a key to a number of other controversial defector on spy cases in which the United States has put great faith.

Nosenko gave the CIA information about other spy cases and defectors that may have seriously misled the United States, his doubters believe, allowing the Soviets to continue other, more damaging intelligence operations against this country.

One case cited by Nosenko's doubters involves the earlier testimony of another Soviet defector whom the CIA officials who doubted Nosenko tended to believe. That earlier defector said that there was a KGB "mole" in a high post inside the CIA. Nosenko said there was a mole, but then gave evidence suggesting that it was pheral, utterly unimportant figure, Nosenko's doubters wonder if he drew suspicion away from a much more important "mole".

Another important case involving Nosenko was that of "Fedora," a Soviet official at the United Nations who worked secretly for U.S. intelligence for many years. Fedora staked his credibiltiy on a confirmation of key elements in Nosenko's original story to U.S. officials that Nosenko himself later admitted were untrue, according to Epstein and other sources.

Some intelligence offiicials believe this episode showed that Fedora-who was given that code name by the FBI-was actually another Soviet plant. But Fedora is still highly regarded inside the intelligence community, authoritative sources told The Post.

Epstein disclosed the existence of Fedora in his book, and cited elements of the case against his reliability in the book, and cited elements in the book and in an interview with New York Magazine.

Fedora apparently has provided the United States with extensive information on Soviet intelligence activities and government policy for years: He is described by Epstein as a former official at the United Nations.

Superficially, his career paralleled that of Arkady Shevchenko, the Soviet U.N. official who decided recently that he would not return to the Soviet Union because of unspecified disagreements with his government.But authoritative sourves said Sevcheko was not the same person as Fedora.

The possiblity remains that Shevchenko's defection is somehow connected to the Fedora case or the publication of Epstein's book revealing the existence of Fedora.

The Nosenko story has filled thousands of pages of CIA reports already, and cannot be concisely summarized. Epstein and former CIA officials who encouraged his skepticism about Nosenko's legitimacy have noted these aspects of the tale:

The timing of his defection and the message he carried were both suspicious. Nosenko first made contact with the CIA in Geneva in 1962. At that time he offered to spy for the United States in Moscow, Americans who dealt with him then were suspicious of NOsenko, but waited to see what would happen.

In January 1964, Nosenko reappeared in Geneva accompanying a Soviet disarmament delegation. He again made contact with the CIA, and announced that he had personally supervised Oswald's KGB file during the perios before Kennedy's assassination. Moreover, Nosenko claimed, he had been assigned to review the Oswald file at the time of the assassination to satisfy senior Soviet officals that there could be no connection between the shotting of Kennedy and Oswald's sojourn in Russia.

Nosenko told the CIA that Oswald had absolutely no connection with the KGB during his stay in the Soviet Union, and that he was not debriefed or questioned by any offical Soviet agency.

At his 1962 meeting with the CIA, Nosenko said he would spy for the United States but would never defect.(Why he wanted to do even spying of this kind was never made clear, according to a CIA offical who took part in the 1962 meeting.) But in 1964, in the course of a series of clandestine meetings with U.S. officals, Nosenko said he had to defect at once because he had received a cable from Moscow recalling him. Nosenko said he feared the KGB had learned of his contacts with the CIA, at which point the Amercians agreed that he could defect.

CIA officals suspicious of Nosenko faulted this story on several counts. THey doubted that an officer with Nosenko's career history could have served in all the capacities he claim. They doubted that the Soviets would ignore a man like Oswald who came to Russia after serving in the U.S. Marines announcing his intention to "defect"-they traditionally at least question such "defectors". And they were suspicious of the fact that such a well-placed Soviet defector would suddenly materialize at the very moment the Warren Commission in Washington was investigating Oswald's past, including his stay in Russia.

In questioning of Nosenko, these skeptical CIA officials found what they regarded as grave contradictions in his story. A key break came when the National Security Agency concluded from its ability to at least partially eavesdrop on Soviet diplomatic communications that there was no telegram from Moscow to Geneva recalling Nosenko, as he had claimed.

Confronted with this intelligence, Nosenko admitted he had lied about the telegram to make his plea to be allowed to defect more persuasive.

(Fedora, the FBI's agent within the Soviet apparatus at the United Nations, had previously confirmed that Nosenko did receive a recall telegram in Geneva,though now Nosenko admitted he did not.)

Nosenko also admitted under questioning that he was not a colonel in the KGB, as he had claimed, but a captain. He said he had lied to exaggerate his importance in the Americans's eyes.

Then why, the questioners, persisted, did he bring with him a purported KGB travel document identifying him as a colonel)? Was this part of a "legend" the KGB created for him before his "defection"?

No, said Nosenko, just a clerical error.

These and other questions about Nosenko were raised in a 900 pages of report and analysis prepared on the case by Bagley, deputy head of the Soviet Russia Division. Bagley had participated in the Nosenko case from the time of his original 1962 contact with the CIA through his prolonged detention and interrogation. Because Nosenko never "broke" under questioning, Bagley concluded that there was no firm proof that he was a KGB plant. But he compiled massive circumstantial evidence which he, Angleton and others felt pointed toward that conclusion.

The officals involved in the Nosenko case then agreed that they could do no more than declare his bona fides in doubt, give him a new identity and money to live on, and go about their business.

But later, acording to 'Epstein's book, the case was reopened. Another officer named Bruce Solie rexamined the evidence and concluded that Nosenko was legitimate. He was rehabilitated and again became a consultant to the CIA.

Reached by telephone, one former CIA director, Richard Helms, said the Nosenko case had indeed divided the agency, and that many people involved in the case were never satisfied about his credentials. Another former director, William Colby, said: "As I understand it, the Nosenko thing was very carefully looked at by the leadership of the agency and they came down on the position that he was basically reliable."

Epstein was astounded to discover that the Soviet government apparently believed the same thing-or said it did.

While researching his book, Epstein said in an interview, he wanted to ask the Soviet government for permission to travel to Minsk, the Soviet city where Oswald lived, to talk to people who knew Oswald there. The Readers Digest, which sponsored and published Epstein's book, contacted Ambassador Anatoliy F. Dorbrynin, and in time Epstein's F. Dorbrynin, and in time Epstein met with an offical named Igor Agu, who identified himself as a press attache in the Soviet embassy in Washington.

Agu heard Epstein's request and for a time they negotiated back and forth. Finally Agu told the author that it would not be possible for him to go to Minsk.

There he offered a suggestion, nothing at the time they negotiated back and forth. Finally Agu told the author that it would not be possible for him to go to Minsk.

Then he offered a suggestion, nothing at the time that perhaps he was exceeding his authority as a Soviet press attache, but recommending nevertheless that there was one man in America who knew all about Oswald's life in Minsk: a defector named Yuri Nosenko.