Federal prosecutors here have decided to break a longstanding policy by investigating members of Congress identified as alleged cocaine users during a probe of a drug distribution network in the Capitol, a well-placed source said yesterday.

The source, a federal law enforcement official familiar with the investigation, said that Justice Department officials have told the U.S. Attorney's office here that because congressmen are public officials, prosecutors must pursue the allegations.

Possession of cocaine is a federal crime, but prosecutors here usually concentrate on drug dealers, not users, according to law enforcement officials.

The source said the allegations involving the congressmen remain unsubtantiated. "I don't know where we're going to get the proof at this point. We certainly can't have an undercover operation," especially since the investigation has been widely publicized, the official said.

"I think Justice officials were concerned with the appearance of not pursuing congressmen in the drug area, even though it is clearly not ordinary" to pursue users, the source said.

The decision ends a month-long dispute between some investigators anxious to pursue the allegations against congressmen, and prosecutors who argued that the lawmakers should not be treated differently.

The federal grand jury investigating the alleged ring had been concentrating on suspected ring members and Capitol Hill aides allegedly used to deliver the drugs to government offices.

Rep. Robert K. Dornan (R-Calif.), who, after receiving complaints from investigators called last week for prosecutors to investigate the congressmen, said yesterday that he was pleased with the decision.

Dornan, a member of the House Select Committee on Narcotics Abuse and Control, had argued that congressmen must be held accountable for using cocaine because they pass the laws that determine the legal status of such drugs.

Last year, Dornan permitted an undercover D.C. police detective to pose as one of his aides, and sources said that information developed by the investigator was turned over to a joint federal and police task force that arrested three alleged members of the cocaine ring in April.

Dornan said yesterday that D.C. police detectives involved in the probe have told him that the investigation has turned up the names of one senator, six congressmen and two former House members as cocaine users. He quoted investigators as saying that three separate sources had implicated each of the nine.

Dornan, who last week requested a separate investigation by the narcotics committee, said yesterday that he no longer sees a need for such an inquiry, though he wants the panel to hold hearings on possible congressional drug use.

The cocaine investigation is separate from other probes being conducted by the Justice Department and the House ethics committee into allegations that some congressmen solicited and had sex with teen-age congressional pages, sometimes in return for possible promotions.

Last week, Leroy Williams Jr., an 18-year-old former page who is the primary source of the allegations, failed a lie-detector test administered by the FBI when he was asked about his alleged sexual relations with congressmen, according to Williams' lawyer and an FBI official.

Williams has told the FBI that he had sexual relations with three congressman and that he arranged for a senator and a congressional aide to have sex with prostitutes.

Two of the congressmen, the senator and the aide have told The Washington Post that the allegations are untrue. The third congressman has not been reached for comment.

The House ethics committee has hired a former assistant U.S. attorney from Philadelphia, Donald A. Purdy Jr., to supervise its inquiry into the alleged sexual misconduct and drug use on Capitol Hill.