What started out as an effort by Gov. Charles S. Robb to put some teeth in Virginia's weak conflict-of-interest law appears to have boomeranged, and the big losers may be Northern Virginia localities.

That preliminary assessment surfaced this week in the wake of an unusually blunt letter that Democratic Attorney General Gerald L. Baliles sent to Robb, informing him that he is powerless to strengthen the state's new ethics law on his own.

Not only is Robb forbidden from issuing an executive order that imposes more stringent requirements than are contained in the law, Baliles wrote last week, but local governments now are also barred from "acting in this area."

The clear implication, some local officials now fear, is they have lost whatever authority they previously had to adopt tougher standards than the state. In addition, some ethics laws in Fairfax and Arlington counties that are already on the books may now be invalid.

"That's putting us back into the dark ages," said Fairfax Supervisor Audrey Moore, referring to the Baliles letter.

Baliles' conclusion is disheartening to Moore and others because it appears to reverse opinions by attorney generals going back to Andrew Miller in 1971. It also confirms the suspicion held by many that when the General Assembly, after considerable furor, passed its new conflicts law last March it was far less of a stride forward than state legislators had proclaimed at the time.

To some extent, Fairfax officials say they were aware of the implications of the new state law and were drafting legislation to bring the county code into compliance with it.

According to a recent memo by County Executive J. Hamilton Lambert to the Board of Supervisors, the impact of the changes cuts both ways. In some cases, county disclosure requirements would be slightly strengthened. In others, however, the county law will be made weaker and less information will be available to citizens about their local officials.

For example, existing county law requires government officials, such as county planning board members, to disclose if they are the officers or directors of any corporation. The new superseding state law says only that paid positions must be reported.

More significantly, the existing county law compels county supervisors and other officials to disclose all their business investments valued at more than $1,000. But the new state law says business investments must only be disclosed when the government official owns more than 3 percent of the outstanding stock in a company or receives more than $10,000 in annual income from it.

Consider one hypothetical example: County Supervisor X owns $50,000 worth of stock in a large firm applying for a county-backed industrial revenue bond to finance an expansion of its plant. Under existing law, the public would at least know of the supervisor's interest before he votes on the bond application. Under the new state law, if the company is large enough, the supervisor's economic interest could be legally shielded from public view.

Fairfax has even stricter disclosure requirements for campaign contributions from rezoning and cable television applicants because of the high stakes involved and the potential for abuse. The question raised by Baliles letter is whether those too might be struck down.

"It would not be a good thing for the zoning process in the county," Moore said. "What it would mean is that I could take a campaign contribution from a developer of $100 or less this year and next year I wouldn't have to disclose it."

In Arlington, which also has stricter requirements on zoning cases, County Board member John Milliken said he would be "deeply disappointed" if those requirements are now overturned. But he added, "I would hope we could maintain those standards as a matter of practice if not as a matter of law."

All this is not what Robb had in mind when he asked Baliles to look into the conflicts issue just a few weeks ago. Robb was disturbed when he discovered in July that his chief mine inspector, Harry D. Childress, could own nearly $19,000 in stock in the state's largest coal company and not be in violation of conflict-of-interest requirements. Robb, sensitive to what he calls the "appearance of impropriety," had expressed an interest in imposing his tougher standards on state officials.

But Baliles has ruled that out and, in doing so, blamed the General Assembly. According to Baliles, it was a legislative conference committee that, in the final days of this year's session, deleted existing language empowering other governmental entities, such as governors and local governing bodies, to go beyond what was in the state law.

Critics, however, are apt to remember it was Baliles who drafted the new conflict-of-interest law in the first place and then publicized it as one of the major substantive achievements of his tenure as the state's highest legal officer. It is an achievement that Moore and others are now wondering if the state could have done without.