The Justice Department said yesterday it may ask the courts to dismantle court-ordered school busing plans affecting several major U.S. cities.

"We have said all along that the Justice Department is opposed to relying on mandatory busing as a remedial technique to desegregate public schools," said William Bradford Reynolds, head of the department's Civil Rights Division. "That remedy has failed to work in school district after school district."

Reynolds said the department will not initiate efforts "to undo existing decrees. Where a school board seeks to modify a busing plan that is not working and requests our support, we will of course give that request serious consideration and where appropriate we might well support modification in court."

Sources in the department said the change could lead to reopening of desegregation cases in Boston, St. Louis, Cleveland, Memphis, Detroit and Denver.

Thomas Atkins, general counsel for the NAACP, which has been a party to many of the desegregation lawsuits, said, "The Justice Department . . . has now opted to take a negative role in civil rights generally and school desegregation specifically.

"We will consider it an all-out assault," he said. "It seems to me they're signaling an intent to join the law violators. . . . The question arises as to whether the Civil Rights Division of the Justice Department has any continuing reason for existing."

Last month the department asked a federal appeals court to allow reconsideration of a school desegregation plan in Baton Rouge, La., on the grounds that white students were fleeing the public school system. It was the first time the Reagan administration has asked an appeals court to restrict a busing plan already in effect.

Until now, the government has rejected school board arguments that white flight was a justifiable reason for rejecting mandatory busing as a remedy for school segregation.

Reynolds said the department has already "spoken with some parties dissatisfied with outstanding busing decrees, and possible Justice Department participation in an effort at modification is being considered with respect to several school districts."

The Boston Globe reported in June that the Justice Department was studying the mandatory busing plan in effect there since 1974. Ian Forman, a spokesman for the Boston School Committee, said yesterday that when the busing plan went into effect, whites made up 70 percent of the enrollment in the public schools. Today, the system is 30 percent white with blacks and other minorities making up the other 70 percent.

The Justice Department has also been asked to intervene in the St. Louis busing case. The St. Louis Post-Dispatch reported on Sunday that the department is already looking into the busing order there. The newspaper said that Rep. Richard A. Gephardt (D-Mo.) and Missouri Attorney General John D. Ashcroft have asked the Justice Department to intervene.

If the department decides to intervene in a particular order, it would probably file a request with the judge in the case asking him to modify his order so school districts could use remedies other than busing to achieve desegregation. The department could also join in an appeal of the order.

The judge would be free to reject the Justice Department request, and civil rights lawyers say many judges have ruled that the Supreme Court requires busing if it is necessary for desegregation.

In a major decision in 1971, called U.S. vs. Scotland Neck Board of Education, the Supreme Court ruled that white flight may be a cause of concern, but it "cannot be accepted as a reason for achieving anything less than a complete uprooting of the dual public school system."

President Reagan has been a vocal opponent of mandatory busing, and the issue has been a prominent one on his social agenda. Earlier this year, the administration announced it was supporting legislation to bar federal judges from requiring that a student be bused more than five miles or 15 minutes from his home. The bill was passed by the Senate but is still pending in the House Judiciary Committee.

Reynolds has said before that he would not rule out the possibility that the department would attempt to reopen school desegregation cases but he has expressed reservations about it. Yesterday's announcement followed an article Sunday in the Post-Dispatch saying that Justice was considering intervening in the St. Louis busing case.

Neither the Justice Department nor the NAACP had any figures on the number of busing orders in effect nationwide or the number of children affected. There also were no figures on the number of school districts involved in appeals of federal desegregation orders.