ASHINGTON, Jan. 21 — Narrowly divided on a question of federal versus state regulation, the Supreme Court on Wednesday upheld the Environmental Protection Agency's authority to insist that factories and power plants use the "best available" anti-pollution technology and to block construction of those that, in the agency's view, do not.

The 5-to-4 decision came in a Clean Air Act dispute between the federal agency and Alaska over conditions for allowing the world's biggest zinc mine to expand its operations. The E.P.A. had overruled Alaska's acceptance of the technology that the Red Dog Mine proposed to use to control the additional pollutants that the expanded facility would emit.

The ruling has implications for the new generation of coal-fired power plants being planned across the Rocky Mountain West. These will require permits under the same provisions of the Clean Air Act that the court interpreted Wednesday.

As an affirmation of the federal agency's authority over the states, the E.P.A.'s victory was in some respects an awkward one. The solicitor general's office vigorously defended the agency's authority when the case was argued in October. But a month later, the Bush administration abandoned more than 50 investigations into violations of the Clean Air Act, a sharp change in enforcement policy that rejected making use of the agency's full authority.

The agency's official response to the ruling appeared to reflect the situation's inherent ambivalence.

"We're reviewing the opinion," Cynthia Bergman, an agency spokeswoman, said, "and while we're pleased, we continue to believe that states have the primary responsibility for the enforcement of our environmental statutes."

Vickie Patton, a lawyer for Environmental Defense, an advocacy group that filed a brief in the case, said the decision "highlights the stark contrast" between the power the court has affirmed and the current position of the executive branch in "dramatically scaling back the use of these vital legal protections."

The Red Dog Mine, 100 miles north of the Arctic Circle, is the biggest private employer in northwest Alaska. In 1996, the mine owner, Teck Cominco Alaska Inc., proposed expanding production by 40 percent. The plan required a special Clean Air Act permit because Alaska, having met the national standards for nitrogen dioxide pollutants, is required by the statute not to allow "significant deterioration" of its good air quality.

The question for the Supreme Court was whether federal or state environmental officials should have the final word on whether the mine's proposal to control its new emissions was the "best available control technology" that the law requires.

Alaska's Department of Environmental Conservation had accepted Cominco's proposal to install an anti-pollution technology known as Low NOx, for low nitrogen oxide, as "logistically and economically less onerous" than an alternative that, while more expensive, was more efficient. The E.P.A. vetoed that decision on the ground that the state had not justified its refusal to insist on the alternative as the best available.

The United States Court of Appeals for the Ninth Circuit, in San Francisco, upheld the E.P.A. In Justice Ruth Bader Ginsburg's opinion affirming that decision, the Supreme Court said Congress had "expressly endorsed an expansive surveillance role for E.P.A.," which the agency carried out properly when it found Alaska's action to be unreasonable.

While the case, Alaska Department of Environmental Conservation v. Environmental Protection Agency, No. 02-658, presented a statutory rather than a constitutional issue, it had overtones familiar from the court's debates over the allocation of federal and state power.

The dissent by Justice Anthony M. Kennedy accused the majority of "relegating states to the role of mere provinces or political corporations, instead of coequal sovereigns entitled to the same dignity and respect." But the states were divided in this case, with a group of mostly Western states siding with Alaska, and Northeastern states siding with the federal regulators. States including New York, New Jersey and Connecticut told the court that E.P.A. oversight "provides a necessary backstop" to clean air enforcement. These states' attorneys general recently declared that they intend to bring their own enforcement suits if the E.P.A. will no longer do so.

Justices John Paul Stevens, Sandra Day O'Connor, David H. Souter and Stephen G. Breyer joined the majority opinion. Chief Justice William H. Rehnquist and Justices Antonin Scalia and Clarence Thomas joined the dissent.