Gov. Charles Robb, hoping to head off a third-party U.S. Senate race by his chief black supporter, today ordered the state tax department to refuse tax exemptions for segregated schools.

The Democratic governor said he issued the regulation, which will take effect immediately, to distinguish Virginia tax law from a Reagan administration policy change in January that could allow tax breaks for more than 100 racially discriminatory academies around the country, including at least 11 in Virginia.

Because the federal change has yet to take effect, state tax officials said the regulations will have no immediate impact. The timing of the Robb move, moreover, was quickly interpreted as an effort to placate Sen. Douglas Wilder (D-Richmond), whose criticism of party leaders on racial issues has brought him to the edge of an independent U.S. Senate candidacy that is threatening to doom Democratic chances at the polls this fall by siphoning off black votes.

The Robb regulation is identical to a Wilder bill that was defeated in a House committee on the last night of the recently concluded 1982 General Assembly session.

"It's a total political move by Robb," said Del. W.R. (Buster) O'Brien (R-Virginia Beach). "He's trying to make peace with Wilder and keep him out of the Senate race. If Wilder doesn't run now, he will have pulled a good poker bluff."

After the defeat of his bill last month, Wilder, the only black state senator, stormed out of the committee blasting Democratic party leaders for their "insensitivity" to black concerns. Since then he has talked publicly of running, a candidacy that many state officeholders say could sink the expected party nominee, Del. Owen Pickett (D-Virginia Beach) and guarantee the election of Republican Congressman Paul Trible of Newport News.

Wilder said he would make up his mind about whether to run for the Senate after consulting with black leaders around the state this weekend, but he danced around questions relating to the impact of Robb's regulation on segregated schools.

"To be brutally frank, this will not make any difference in my personal opinion," he said from his Richmond law office. But he added that the Robb regulation could effect the thinking of other black leaders. "And if these people suggest to me that they would not like me to run, then I may not."

Pickett and Wilder met privately for 45 minutes yesterday to discuss the upcoming race. Pickett said they both agreed that a third candidacy by Wilder could be a "deciding factor," but that he did not urge Wilder to stay out of the race.

Aside from its politically symbolic meaning, the actual effect of the Robb regulation was unclear. State tax officials said they had no idea how many schools in Virginia discriminate on the basis of race and lacked the means to find out.

"It's going to be very difficult to enforce," said Edward Dore, director of the state's tax policy division. "Whenever we get any information about a school that indicates that it discriminates, I guess we'll send one of our investigators out to find out what they can. But we'll have to rely on information coming to us."

Until today, the Virginia tax code simply tracked federal tax exemptions. If a particular expense qualified as a deduction on the federal tax form, it automatically qualified as a deduction under state tax law.

The pressure to spell out a specific state policy regarding segregated academies began after the Reagan administration announcement that it would no longer enforce a longstanding Internal Revenue Service regulation forbidding deductions to persons who contribute to private schools that were found to discriminate.

Since then, however, the tax status of segregated academies has been clouded by a pending Supreme Court case and legislation in the Congress that would replace the suspended IRS regulation with a federal statute. There also has been a U.S. court injunction preventing the administration from enforcing the policy change.

Despite the confusion, Robb's regulation was angrily criticized by fundamentalist ministers who operate hundreds of church schools in the state.

"It's a horrible thing," said the Rev. Bud Calvert, moderator of the Virginia Assembly of Indpendent Baptist Churches." . . . If they do this on race, then they could do it to us with homosexuals or any other group."