Almost from the day they took office, Reagan administration officials have talked about the need to relax the federal government's principal ethics law, contending that its tough provisions are driving top-quality businessmen from government service.

But, the General Accounting Office concludes in a new report, there is little evidence that the 1978 Ethics in Government Act has been a major barrier to federal employment. In fact, it said, the law's provisions are only one of many factors, including low pay and the personal rigors of life in Washington, that have caused problems for federal recruiters.

The report undoubtedly will be used by supporters of the Office of Government Ethics, a 21-employe bureau in the Office of Personnel Management, when it comes up for a five-year reauthorization this year.

"What we've found is that the law has been used as a scapegoat," said David R. Scott, the office's acting director. "The classic examples are the people who go through the nominating process and then find that they have trouble with the FBI check or the Internal Revenue Service check. So the most graceful thing to do is bow out and say it's because of the Ethics in Government Act."

In April, E. Pendleton James, then assistant to the president for personnel, called the ethics law the "chief obstacle" to businessmen who might want to enter government.

That comment, and others by White House counsel Fred F. Fielding and some Justice Department officials, have raised fears among officials of Common Cause, the self-styled citizens' lobby, and some senators that the administration may be preparing a new assault on the ethics law in this session.

Last week, for example, Scott appeared to retreat from his previously expressed support for the law's strict public disclosure requirements. He told the Senate Governmental Affairs oversight subcommittee that the "need for and efficacy of public versus confidential financial statements . . . should be considered by the Congress."

"It's disturbing," said Ann McBride, a vice president of Common Cause, which lobbied for the 1978 law. "Here in the midst of everything that's going on at EPA and conflicts of interest, you'd think they'd be a little more sensitive to the need for disclosure . . . ."

Fielding has talked about trying to make financial disclosure statements available only to congressional committees, and the Justice Department's Office of Legal Policy said last year that it is studying such a change.