A longtime Washington restaurateur who has pleaded guilty to drug conspiracy is expected to testify at D.C. Mayor Marion Barry's drug trial that he supplied cocaine to Barry on a regular basis, and that Barry used drugs frequently -- sometimes two or three times a day -- for at least three years, according to sources familiar with the case.

The sources said that Hassan H. Mohammadi, 35, a close friend of Barry's, told authorities that he was the mayor's main cocaine supplier. He could be the prosecution's most damaging witness against Barry, sources said.

Barry has told associates that he is particularly concerned about Mohammadi's testimony. Mitchell Rogovin, an attorney for Mohammadi, declined to comment yesterday, and also said Mohammadi would have no comment. Mohammadi could not be reached.

Mohammadi's testimony could be a critical ingredient in prosecutors' attempts to prove that Barry was a chronic drug user, sources said. One of the 14 counts against Barry is a charge of conspiracy to possess cocaine from 1984 until earlier this year.

Mohammadi told prosecutors that on several occasions he gave Barry cocaine in a private room in the M Street NW restaurant he owned until recently, the Pardis Cafe, at Mohammadi's home and at other restaurants and apartments, according to sources. Mohammadi has told authorities that on numerous occasions, Barry would arrive at Mohammadi's restaurant and walk directly to one of the private rooms, the rear door or the restroom to pick up the drugs, sources said.

At times Barry would use some of the cocaine right away, and on occasion he would return to the restaurant or to Mohammadi's home later the same day for more drugs, Mohammadi has told prosecutors. Sources said that, according to Mohammadi's account to investigators, Barry would sometimes drop into the restaurant during slow times, before or after lunch hour, in the middle of the afternoon or late in the evening. Occasionally Barry would pick up cocaine on his way to the airport en route to conferences, government meetings and vacations, Mohammadi told prosecutors.

Authorities have investigated a $190,000 D.C. lottery board contract issued last year to a company, Media Productions Inc., set up by Mohammadi.

Mohammadi has been talking to authorities off and on for months, sources said. But the discussions were disclosed officially only in late May, when Mohammadi pleaded guilty to conspiracy to possess cocaine as part of a plea agreement in U.S. District Court here.

The plea agreement, filed in connection with the Barry case, is sealed. According to sources, his cooperation was a major break for prosecutors, significantly bolstering the criminal case.

Federal investigators have corroborated parts of Mohammadi's account by gathering information from other alleged drug dealers, who also gave similar accounts to The Washington Post.

Mohammadi has been a law enforcement informant periodically for years.

He was arrested in September 1982 in connection with the bribery of a U.S. Immigration and Naturalization Service official in exchange for issuance of fraudulent permanent resident documents for Iranian nationals. He transported cash and visa documents between the ringleaders and the INS official, court papers said.

The next month, in a plea agreement with the government, Mohammadi pleaded guilty to conspiracy and fraud, and started working closely with the FBI and other law enforcement agencies, court papers said. Mohammadi met with investigators more than 40 times during that period, according to court documents. His testimony helped put several of his former friends in jail. Barry's attorney in the drug trial, R. Kenneth Mundy, was an attorney for one of the defendants.

Robert S. Tignor, a prosecutor with the Justice Department's public integrity section, said in a letter to the judge in July 1983 that Mohammadi had provided "substantive" information to investigators and had been a "very articulate and effective witness" at trial. Mohammadi also "has provided useful information to the FBI" in two other investigations. "We are satisfied that Mr. Mohammadi has testified truthfully and that he has cooperated fully with the government," the letter said.

Mohammadi received a five-year prison sentence, which was suspended, placed on probation for three years and fined $10,000.

In 1986, Mohammadi acted as an informant for law enforcement agencies in supplying information against other businessmen in the local Iranian community, sources said. Mohammadi told investigators that some of them, to whom he owed money, were trying to strong-arm him, the sources said.

In May 1989, Mohammadi was mentioned in testimony before a federal grand jury in Alexandria that was investigating an alleged drug ring. Mohammad Sia Pasikhani, a former drug dealer, told the grand jury that Mohammadi obtained cocaine "for the purpose of entertaining some guests," according to a transcript of his testimony. Sources said Pasikhani later told authorities that on various occasions he gave Mohammadi cocaine that Mohammadi then gave to Barry.

Barry has told The Post that he first met Mohammadi when he was an employee of the Iranian embassy under the Shah of Iran in the late 1970s, and that he and Mohammadi became friendly several years ago when the mayor attended a party at the Pardis Cafe. Barry said that Mohammadi and his wife, Yasaman Rowhani, have been to his house, and that "I've been to his house with five or six people," but never alone.

Staff writer Susan Schmidt contributed to this report.