For more than 25 years, an operation code-named "Solo" has been one of the FBI's most jealously guarded secrets. The information it produced was sometimes deemed important enough to be handed directly to presidents.

"There's no doubt that the bureau considered Solo its greatest intelligence coup," said one official familiar with the long-running operation. "On a scale of 1 to 10, Solo was a 10," said another.

Solo was the protective cover for two brothers recruited and paid to infiltrate the highest echelons of the American Communist Party. In the 1950s, sources said, one became the courier of up to $1 million a year in secret contributions by the Soviet Union to sustain the struggling party. This was cited to support J. Edgar Hoover's claims about the communist threat within.

More important, knowledgable intelligence officials said, the two men traveled abroad and were accepted as confidants by communist leaders, enabling them to return to their FBI handlers with news about the intentions of communist-bloc officials. One even had his picture taken with Soviet leader Leonid I. Brezhnev. For years, they were considered the FBI's most important counter-intelligence assets.

Now, in a book about the bureau's smear campaign against Martin Luther King Jr., David J. Garrow, a University of North Carolina professor, exposes Solo for the first time and identifies the two brothers as Morris and Jack Childs.

An FBI spokesman had no comment on the disclosures in the book. But The Washington Post independently has verified the Childs' role as FBI informants and learned something of their activities. Jack Childs died last year and Morris' current whereabouts could not be learned.

The operation was considered so successful that as the brothers got older officials considered trying to train others to take their place. It is less clear how valuable the brothers' information was to national security, especially in recent years when the American Communist Party was considered less of a threat.

And there is also the usual uncertainty about whether the Soviets ever caught on to the Childs' double-dealing and could have used them to send back disinformation to the FBI. Sources said, for instance, that the CIA had serious doubts about the authenticity of the FBI's intelligence regarding the Soviet financing of the American party.

The Solo operation also raises the ironic possibility that the U.S. government condoned illegal Soviet funding of the American Communist Party for years just so it could keep tabs on the party's finances.

One former intelligence officer said Jack Childs used to pick up the Soviet cash from a diplomat attached to the Soviet United Nations mission in New York. Other officials said that Solo enabled the FBI to discover where the Communist Party kept its money and how it used it, and that American party boss Gus Hall "was bullish on America," investing some of the Soviet money in the stock market and apartment buildings.

During the Ford administration, some officials felt Solo had outlived its usefulness, and consideration was given to publicizing the operation. But the idea was rejected.

A Soviet embassy spokesman yesterday flatly denied the allegation about Soviet funding of the American Communist Party. "We don't engage in this type of activity," he said. "We don't finance the Republican Party, the Democratic Party or even the Anderson campaign."

Lem Harris, a Communist Party member from Connecticut who knew the Childs brothers, said yesterday he thought it unlikely that they were FBI informants. If the FBI discovered the Soviets funding the American Communist Party, he said, "I'm sure they would have trumpeted the news, because that's just what Hoover would have wanted." American Communist Party leader Hall is out of the country and his office in New York did not return a call yesterday.

Garrow said he came upon Solo while trying to learn why the FBI carried out such an extensive campaign to discredit King. He found that agents had been told by Jack Childs that Stanley Levison, a King confidant, was an active Communist Party member in the early 1950s. That led to wiretaps of Levison and eventually taps of King, too, authorized by Attorney General Robert F. Kennedy.

The book, "The FBI and Martin Luther King Jr.: From 'Solo' to Memphis," traces the bureau's unsuccessful effort to leak descriptions of the civil rights leader's sex life to reporters during the early 1960s. Garrow also names an informant from the Southern Christian Leadership Conference office in Atlanta who allegedly spied on King for the FBI in the mid-1960s.

The congressional committees that investigated FBI spying and the 1968 King assassination were briefed about Solo, sources said, but were never told the identity of the key operatives. The Justice Department's 1977 internal investigation of the FBI role in the King murder investigation notes only that the characterization of a King adviser as a communist "was provided by sources the bureau considered reliable."