A mysterious Soviet diplomat named Igor who volunteered his services to the Central Intelligence Agency in a phone call to the home of Richard M. Helms may hold the key to the disappearance of a Soviet defector named Nicholas G. Shadrin, Time magazine reported yesterday.

The Washington Post revealed the first details of Shadrin's disappearance last year. His real name was Nikolai Artamonov, and he defected to Sweden, then to the United States in 1959. He said he was a Soviet naval officer on temporary duty in Poland at the time. He went to work for the U.S. government in Washington.

In 1966 Shadrin became a "double agent" for the Federal Bureau of Investigation, pretending to go to work for the KGB (Soviet secret police) in Washington (or so the FBI thought) while actually working for U.S. intelligence. In December 1975, on a secret mission to Vienna to meet his KGB contacts, Shadrin disappeared.

According to Time's account, U.S. officials encouraged Shadrin to become a double agent in 1966 "despite considerable misgivings on his part," to help the KGB agent name Igor who had volunteered to work inside the KGB for the United States.

Accordint to Time, sometime in 1966 Igor telephoned the home of Helms and talked with his wife. (Time gives no date for this call. In June 1966, Helms was promoted from director of covert operations to CIA director.) Igor said he wanted to work for U.S. intelligence.

Mrs. Helms "turned Igor over to her husband, who in turn passed him on to U.S. counterintelligence operatives."

"Igor told the Americans that he could possibly get a higher post within the KGB," Time's report continues. "He said he would have a better chance of this is he could recruit Shadrin as a double agent."

If this version is correct, it would fill a vast hole in previously available accounts of Shadrin's career and disappearance. Shadrin was a Soviet defector allegedly sentenced to death by a Soviet court in absentia, who once testified to a congressional committee here that the Soviet Union had secret plans for a sneak attack against the United States. The hole in the story was, why would a man with that history agree to work for Soviet intelligence in 1966?

(Rearched by telephone last night, Helms confirmed that a Soviet official had once called his residence and talked with his former wife. Helms said he passed this on to the "relevant officials," and "I never much followed it after that."

("There's no reason why it couldn't have been Igor," Helms added, but he said he did not recall that name.)

According to Shadrin's wife, Blanka Ewa Shadrin, a dentist living in McLean who defected from Poland with her husband in 1959, she knew nothing about her husband's double-agent role until 1975, when he disappeared in Vienna. She could offer no explanation for his previous behavior.

Some sources in the intelligence community have suggested that Shadrin actually was a Soviet plant from the beginning, in 1959. Other sources felt he was a patriotic American (Congress granted him citizenship in the 1960s) prepared to take great risks for his new homeland.

Time's report suggests that the actual phony agent may have been the mysterious Igor. "There is a lingering suspicion in intelligence circles that in going along with Igor's request to recruit Shadrin," Time says, "the U.S. fell for a Soveit plot."

The magazine quotes one U.S. intelligence official as speculating that there might be a connection between the name Igor and the Russian word for a game, "igra."

Time gives no indication of what Igor's relations have been with U.S. intelligence since 1966.

Shadrin disappeared in December 1975. He had had one meeting with two KGB agents, and was last seen setting out for a second rendezvous.

The most common assumption within the U.S. intelligence community has been that the Soviets kidnaped and perhaps killed him.Mrs. Shadrin has held out hope that he is still alive and might somehow be swapped.

Preident Ford wrote to Soviet leader Leonid I. Brezhnev late in 1976 asking about Shadrin. Brezhnev replied in a message passed on to the White House by a Soviet diplomat here. The message was that Shadrin never showed up for the second meeting in Vienna and the Soviets had no idea of his whereabouts.

Last August, after The Post and The Wall Street Journal published reports about Shadrin's disappearance, a Soviet newspaper published an unusual, long account of the case that suggested the CIA had done away with him.