The Carter administration yesterday endorsed busing between a city and its suburbs to desegregate schools in the Wilmington, Del., area, but said it is not departing from a basic policy established under President Ford.

That policy is to seek court orders that eliminate constitutional violations and all of their lingering effects "root and branch" but do not try to achieve" a desirable racial mix," the Justice Department said.

A tentative court-ordered busing plan for the school system in the city of Wilmington and 10 school districts in New Castle County is the first in a large metropolitan area to be supported by the government.

But, the department emphasized, acts of racial discrimination in the Wilmington case were inter-district, were "significant and continuing" and were exacerbated by state and local authorities.

In light of "the practicalities of the situation," the department said, "substantial inter-district reassignment is the only way in which the inter-district violation in this case can be effectively remedied. . ."

The department set out its position in a friend-of-the-court brief filed in the Third U.S. Circuit Court of Appeals.

The brief was signed by Assistant Attorney General Drew S. Days III nine days after taking office as head of the department's Civil Rights Division. Days is a former attorney for the NAACP Legal Defense and Educational Fund.

The brief also had the approval of acting Solicitor General Daniel M. Friedman, as required for any department friend-of-the-court filing in an appeals court.

The position taken by the department in the Wilmington case had been signaled by the administration. On Feb. 13, for example, Attorney General Griffin B. Bell said that the scope of remedies for school desegregation should be no greater than the proved wrongs.

Bell's predecessor in the Ford administration, Edward H. Levi, decided last May to consider the cross-district student reassignment order in the Wilmington area a test case of whether federal judges were ordering too much busing to end school discrimination.

Levi acted shortly after a three-judge panel gave the school districts two years to consolidate and desegregate with massive busing. The city schools are 88 per cent black, the suburban schools 94 per cent white.

Last October, the department said that the panel correctly had ordered substantial busing, but objected to some language in the panel's opinion as "suggestive of an attempt to achieve a desirable racial mix rather than to remedy constitutional wrongs."

The department took the same position yesterday. "The principles governing the remedy in this case are well established," the brief said. It continued:

"The goal of a remedial order in a school desegregation case should be to put the school system and its students where they would have been but for the violations of the Constitution . . .

"A court is not at liberty to produce a result merely because it may find the result desirable. The existence of a violation of the Constitution does not authorize a court to bring about conditions that never would have existed in the absence of official racial discrimination . . .

"The proper approach requires a court to seek to determine, as precisely as possible, the consequences of the acts constituting the illegal discrimination and to eliminate their continuing effects . . .

"The existence of schools predominantly attended by members of one race does not in itself amount to racial discrimination. A properly formulated desegregation decree should not, therefore, be based on the premise that such schools are undesirable, or that each school should have a racial mixture or balance."

The department asked the appellate court to affirm the panel's judgment to the extent that it would overcome the effects of past discrimination by the school authorities. At the same time the department said the court should strike down any steps "based on a goal of achieving racial balance."