The U.S. Supreme Court agreed yesterday to review a ruling that threatened to block the ability of the regional agency that operates National and Dulles International airports to plan expansions or make other major decisions about airport budgets and operations.

The court said it would consider a decision by the federal appeals court here holding unconstitutional the 1986 law that established the Metropolitan Washington Airports Authority.

While the case is under appeal, the lower court ruling limiting actions by the authority cannot be enforced. Had the Supreme Court refused to take up the case, congressional action would have been necessary before the airport agency could make major decisions, lawyers familiar with the case say.

"We're proceeding with business as usual," Edward Faggen, the agency's legal counsel, said yesterday. He said key actions expected to be taken before the Supreme Court rules likely would not be overturned even if the airport authority loses the case. "It's a question of reasonableness," Faggen said.

The authority's board of directors may be asked next month to amend the Dulles International Airport master plan to allow a private firm to extend the Dulles Toll Road. In addition, the authority must approve an operating budget by October.

Attorneys for the anti-noise group that originally questioned the authority's constitutionality say airport authority actions taken this year could be challenged individually if the agency loses the case. Sherwin Landfield, a spokesman for the citizens group, said he has "a fair amount of confidence" that the law will be found unconstitutional and Congress will have to take up the issue again.

A labor union's challenge to the airport authority legislation also is pending in federal court.

In the case the court agreed to hear yesterday, the appeals court said a provision of the law that set up a review board composed of nine members of Congress, with power to veto important actions by the airport board, violated the constitutional doctrine of separation of powers.

The case stems from a lawsuit brought by a group opposed to the planned expansion of National Airport. Under a master plan adopted in March 1988, the airport will be modernized and the terminals and parking facilities will be expanded.

The group challenging the law, Citizens for the Abatement of Aircraft Noise, filed suit in November 1988 contending that the construction would lead to an increase in noise and create safety and environmental problems. The group asked for a court order blocking implementation of the plan on the grounds that the law violates the separation of powers.

The federal government operated the two airports from their construction until 1987, when the regional authority took control.

In transferring power to the regional authority, however, Congress required establishment of the review board, which has veto power over the authority's budget, the issuing of bonds and planning. The appeals court said that structure violates the separation of powers because it resulted in legislators' performing executive functions -- in effect, controlling key decisions about the airports' future.

In asking the high court to hear the case, the airport authority said the ruling would cause "serious disruption" of the airports and "invalidates a creative use of this nation's federalism to resolve successfully a long-festering regional transportation problem."

The state of Virginia agreed, saying that the law crafted a "delicate balance" between the state and federal governments. "It would be ironic if the complaint of a private citizens group were permitted to undo the carefully crafted political framework of sovereign governments," the state said.

The citizens group agreed that the issue was important enough to merit Supreme Court review but said the appeals court ruling was right. The group said Congress, in imposing the requirement for the board of review, intended to "retain congressional control over the airports after their transfer."