U.S. Attorney Earl J. Sibert, who has been the city's chief federal prosecutor for more than five years, is expected to leave his job by July 1, three months before his term expires.

Silbert's early departure here was prompted by a new federal law - effective July 1 - that will severly restrict activities of lawyers who move from government to private practice.

Sources close to Siblert in the U.S. Attorney's office said, however, that he already had reached a decision to leave the Justice Department, after 15 years of service, and that provisions of the new Ethics in Government Act of 1978 changed only the timing.

A cautious and methodical prosecutor, Silbert has carefully supervised the U.S. Attorney's Office, here - the largest in the nation - which handles virtually all criminal cases in the District and represents the federal government in many major civil lawsuits.

It was for his role as an assistant U.S. attorney and chief prosecutor in the trial of the seven original Watergate defendant's however, that Silbert gained national prominence and faced wide criticism.

The seven defendants either pleaded guilty or were convicted of various charges, but Silbert's investigation was attacked as incomplete when it failed to pinpoint prominent White House officials and a cover-up of their involvement in the Watergate matters. Later, Silbert reopened the investigation, but he was eventually replaced by special prosecutor Archibald Cox.

Nevertheless, Silbert withstood harsh criticism in the Senate and after lengthy examination was confirmed to a four-year term as U.S. attorney since January 1974.

During his years as U.S. attorney, Silbert has been known as a strict law and order prosecutor with particular concern for street crime. ". . .No one wants his home broken into, pocket-book snatched or to be robbed," he was quoted as saying in 1974 when he became a court-appointed federal prosecutor. He also has supervised prosecution of several major white-collar criminal cases.

Privately, Silbert was often critical of judges who decided to release some defendants on bail or impose light sentences after convictions. As an adminstrator, Silbert played an influential role in the development of a computerized information system for prosecutors, now used throughout the country, organized a Career Criminal Unit to focus on repeat offenders and put special emphasis on investigation and prosecution of narcotic cases.

He has been immensely popular within the U.S. attorney's office, where assistant prosectors are often heard to call him "the godfather."

Yesterday, Silbert's secretary at the federal courthouse here said that Silbert would have no comment on reports that he plans to leave the office before July 1.

The Ethics in Government Act, which sources say has resulted in the timing of Silbert's decision, would prohibit him for one year from involvement in any case - as a private attorney - which he would have to deal with the U.S. Attorney's Office here. It would further ban him for two years from direct dealings with the government in any cases currently pending in the prosecutor's office.

Silbert would also be permanently prohibited from representing a client in a case where he had "personal or substantial" involvement.

The new rules, effective July 1, pertain to both civil and criminal cases, Bernhardt Wruble, director of the Office of Government Ethics confirmed yesterday.

While Silbert's departure from the U.S. attorney's job has been rumored for months, associates said yesterday that Silbert has kept his exact plan a secret.

But sources said that at a conference of U.S. attorneys last week at the Washington Hotel, Silbert said he intended to leave the office before July 1, because of the Ethics in Government Act.

A Justice Department spokesman said yesterday that Silbert has not submitted his resignation, which would be directed to President Carter.

Silbert is paid $50,000 a year.

Sources said yesterday that Silbert has had discussions with the Washington law firm of Schwalb, Donnenfeld and Bray.

There were additional indications from various sources that Silbert has talked to the Washington law firms of Ginsberg, Feldman and Bress - the late David Bress once was a U.S. attorney - and to Dickstein, Shapiro and Morin. Several former assistant federal prosecutors who are close Silbert friends either work at the firms or were once employed by them.

It was unclear yesterday whether Silbert has reached a decision on his next job. Whether he has a new position lined up or not, if he serves his connection with the government before July 1, Silbert would not be subject to the new restrictions of the Ethics Act.

Silbert has remained on the job as U.S. attorney through the Carter adminstration - and gained praise from the law enforcement community - although the incumbent president could have made a new appointment.

In December 1977, legal and political sources confirmed that a low-key search for a black attorney from the District who would be willing to accept the nomination was unsuccessful. It appeared then that the attorneys contacted then had reached positions in their legal careers that they were not willing to trade for the prosector's job.

The expected search for a new U.S. attorney could be complicated by negotiations now under way between the District government and the Justice Department to combine the U.S. Attorney's office and the Office of the D.C. Corporation Counsel and create a new local prosecutor's office. The corporation counsel represents the city in civil matters and prosecutes violations of the city code such as housing regulations and health and santiation laws.

While those discussion are underway, numerous assistant federal prosecutors have grumbled that the new local prosecutor's office would not have the authority - and prestige - that comes with federal control. Silbert has vigorously opposed the change.

In the District, unlike any other city in the country, the U.S. attorney prosecutes almost all local criminal cases - from murder to simple assault - as well as federal crimes such as bribery and large-scale drug trafficking. CAPTION: Picture, EARL J. SILBERT . . . first Watergate prosecutor