The Reagan administration, invoking a civil rights weapon unused in 10 years, is cutting off federal funding to a Mississippi school district found guilty of racial discrimination.

The cutoff follows an enforcement proceeding begun during the Carter administration.

The issue is basketball. Reacting to complaints that not enough whites were being allowed to play basketball at the high school in New Augusta, Miss., the Perry County school district subsequently fired two coaches, one black and one white.

The coaches then filed a discrimination complaint with the U.S. Department of Education that led to the cutoff.

A federal administrative law judge found that the coaches and black players had been discriminated against.

Ironically, most of the money the district is losing is Title I aid, which is targeted for the disadvantaged, and, according to the school district's lawyer, T. Jack Riley, the loss will hurt black children there most. He said half the district's 1,400 students are black, and about 75 percent of Title I students are black.

Similar cases involving allegations of discrimination have been settled under the threat of suit or a cutoff by the government in recent years.

Clarence Thomas, the Department of Education's chief civil rights officer and President Reagan's choice to head the Equal Employment Opportunity Commission, said yesterday previous administrations usually found cutoffs were "politically not feasible."

He said he would have preferred to try to work out a settlement, if the school district had been willing, but added he had "no problem" with using the cutoff as an enforcement tool. "I don't think anyone wants to cut off funds to school kids, but that was the remedy pursued in this case, however distasteful," he said in a telephone interview.

An administrative law judge ruled last March that the Perry County schools had violated Title VI of the 1964 Civil Rights Act by discriminating against the black basketball players, and the coaches. The judge found that the black coach and his white athletic director, who also coached, were reassigned and finally fired for failing to play enough whites.

The school district refused to appeal and two weeks ago the Department of Education filed notice that the fund cutoff will take place April 5.

Riley, the school district's lawyer, said in a phone interview from Hattiesburg that the district school board is prepared to lose nearly $200,000 in Title I funding, rather than rehire the coaches.

"They want the moon and all the stars," he said of former coach Clevis Breland Jr. and athletic director Michael Wade. Riley said the district had not contested the Education Department's enforcement proceeding because it felt the state supreme court had decided the issue. The court upheld the firings, accepting the district's arguments that the men were insubordinate. However, the federal administrative judge ruled that the state court had not decided the discrimination issue.

It is unclear whether other federal agencies will also cut off funds, including school lunch funds from the Agriculture Department. Thomas said that his lawyers had informed other agencies of the action.

The cutoff comes at a time when the Reagan administration is under attack by civil rights groups for failing to enforce anti-discrimination laws. Earlier this week, a federal judge in Washington found that the Education Department had failed to meet court-imposed deadlines for processing civil rights complaints but declined to hold Thomas and Secretary of Education Terrel H. Bell in contempt of court.

Perry County's time to appeal the cutoff ran out last April. Education Department Undersecretary William C. Clohan Jr. said last night that the cutoff notice was delayed "because I tried to get the board to take action to come into compliance. Termination of federal funds is a very, very radical step. I wanted to make sure we tried to work something out."

In his decision last March, administrative law judge Walter J. Alprin said that Breland's team won the district championship during the 1976-77 school year, using five black players almost exclusively and keeping his white players on the bench. That summer, school board chairman Oscar Mixon and the rest of the board ordered he not be allowed to coach again.

But athletic director Wade, who was given the job, refused to coach, so Breland was reinstated with the agreement he "would try playing more whites, particularly when his team was winning," the judge wrote. The next year some of the blacks were benched. It was a losing season.