The Justice Department's Office of Professional Responsibility intends to conduct a much broader investigation of the administration's handling of the Billy Carter case than previously was believed according to a directive to be issued today.

Any ultimate decision to prosecute high government officials would be handed over to a special prosecutor outside the department if the inquiry finds specific evidence of a possible crime. But the new order gives Michael E. Shaheen Jr., head of the OPR, enough power to conduct an inquiry like the Senate subcommittee's investigation of the matter.

This includes authority to question the president other White House officials, Billy Carter and his attorneys and Justice officials.

Shaheen's office, which monitors the ethical conduct of department employes, began its inquiry immediately after Attorney General Benjamin R. Civiletti's July 25 admission that he had discussed the Billy Carter case with President Carter.

At the time, it was understood that Shaheen would confine his investigation to whether that June 17 discussion could have led to someone warning Billy Carter to register as a foreign agent, thus heading off a possible criminal charge.

The chief attorney on the case, Joel S. Lisker, said the Carter-Civiletti contact had no bearing on his recommendation to proceed with only a civil action against the president's brother.

The new directive for Shaheen's office, signed by Deputy Attorney General Charles B. Renfrew, opens the way for the OPR to study the handling of classified information in the case by Civiletti and national security adviser Zbigniew Brezinski.

Shaheen and his attorneys also have the power to question White House counsel Lloyd Cutler and Billy Carter's lawyers, Henry S. Ruth and Steven Pollak. In short, Shaheen's mandate has been expanded to allow the beginning of a full-scale criminal investigation to see whether anyone in or out of the government interfered in the case. He can conduct grand jury proceedings and grant immunity to witnesses, and in general wield powers given special prosecutors.

Because of the special prosecutor provisions of the Ethics in Government Act, however, these new powers would be limited if Shaheen's inquiry comes across specific information about a possible crime by a high government official. That would trigger the act, and Shaheen's office would then merely conduct a preliminary investigation, without grand jury or immunity powers, according to John Harmon, assistant attorney general in charge of the department's office of legal counsel, who approved the legal aspects of Renfrew's order.

Shaheen still would have the authority to recommend prosecutions of anyone in or out of government who is not covered by the act.

Because Civiletti and Renfrew both were involved in some aspects of the case, Shaheen will present his final report to Solicitor General Wade H. McCree Jr. He also may submit a final report to other department officials and Congress, if he wishes, the directive says.

Some department officials said they were stunned at the broad scope of Shaheen's mandate and power. Renfrew's order says he has authority "to investigate for criminal, civil and administrative purposes, any offenses arising from the activities of Billy Carter in acting as an alleged agent of the Libyan government, including, but not limited to, the conduct of any and all government employes or appointees, or any other persons . . . Shaheen is expected to use FBI agents and department attorneys in his inquiry.

The OPR's procedure of using only affidavits in its investigations was criticized in 1978. That was during a case in which then-attorney general Griffin B. Bell and President Carter were accused of obstructing justice by firing a Republican prosecutor whose office had been investigating Democratic congressmen.

Shaheen's office was set up by former attorney general Edward H. Levi in the mid-1970s. The Justice Department has resisted efforts to replace it with an inspector general who would be subject to presidential appointment.

The OPR has a reputation for independence, heightened most recently by its 1979 annual report, which criticized both Civelletti and the president for failing to act swiftly enough on recommended dismissals of two department employes.

The new directive is to be published in today's Federal Register.