In a strong signal of the Reagan administration's opposition to busing, Justice Department officials are preparing to reverse the department's support of a voluntary busing plan by three Seattle-area school boards.

Sources said the department now plans to file papers with the U.S. Supreme Court backing the state of Washington's ban on busing for desegregation purposes. This is a bold turn away from the department's previous opinion that the state prohibition is unconstitutional.

The U.S. 9th Circuit Court of Appeals accepted the arguments of the local boards and Justice lawyers in ruling last December that the state ban violated the equal protection clause of the Constitution's 14th Amendment. One Justice official said the department is switching sides now because Reagan appointees reviewing the case believe the state powers take precedence.

The Seattle about-face is considered the most visible sign of the administration's attitude toward ways to desegregate the nation's schools. Two weeks ago, Justice gave interim approval to a Chicago desegregation plan that it had strongly criticized as "incomplete" a month earlier. And in Houston, the department decided not to press its contention that a federal court should force a cross-district desegregation plan between the city and its suburbs.

The Seattle dispute began when the three school boards voluntarily adopted a busing program to desegregate the schools. Parents opposing the plan tried to recall four school board members, and, failing that, started a statewide ballot initiative to ban busing. The initiative passed in 1978, and the school boards immediately challenged it. The Justice Department entered the case on the side of the boards in February, 1979.

The Supreme Court must accept the case because the court of appeals overturned a state law, the initiative, on constitutional grounds. Because Justice intervened on behalf of the school boards, it must state its position, which is expected within days.

Robert Reinstein, a Temple University law professor who argued the case supporting the school boards for Justice before the 9th Circuit, called the reversal "incredible." Reinstein said it is ironic that the Reagan administration is dropping its oft-stated support for local government. "I always thought it was like motherhood and apple pie to support a school district who voluntarily wants to desegregate its schools."

Camden M. Hall, attorney for the Seattle school board, said he was "disappointed but not surprised" that Justice is abandoning its support for the board because he had heard the administration was changing its position. "I think busing is an issue whose time has gone," he said, adding that he still considers the state law unconstitutional.

William Bradford Reynolds, assistant attorney general for civil rights, declined to comment on the Seattle filing. But another Justice official familiar with the case said department officials felt the Seattle, Tacoma and Pasco school boards had made race an issue in desegregation by busing, and the state had a right to remove it.

In his first major speech on civil rights in May, Attorney General William French Smith had foreshadowed this philosophy by saying the government would not be seeking busing as a remedy and that its goal "must always be genuinely color-blind state action."

Sen. Slade Gorton (R-Wash.), who argued the state's case while attorney general, introduced an anti-busing bill in May called the "Racially Neutral School Assignment Act."

In the 9th Circuit's majority opinion, Judge Walter Ely made several references to the sanctity of local control of the schools. He said at one point that the ballot initiative barring busing "both creates a constitutionally suspect racial classification and radically restructures the political process of Washington by allowing a state-wide majority to usurp traditional local authority over local school board . . . policies."

President Reagan and Smith have made numerous statements about the need to move government closer to the people. But one Justice official said department officials decided the state interest was more important than the local boards' since there was no finding of constitutional violation on the local level, and no real evidence busing is required to ensure equal opportunity.