WASHINGTON — The Supreme Court unanimously ruled out a federal lawsuit today by states and conservation groups to force cuts in greenhouse gas emissions from power plants.

The court said that the authority to seek reductions in emissions rests with the Environmental Protection Agency, not the courts.

EPA said in December that it will issue new regulations by next year to reduce power plants’ emissions of carbon dioxide, the chief greenhouse gas. The Obama administration has already started controlling heat-trapping pollution from automobiles and from some of the largest, and most polluting, industrial plants.

But the administration’s actions have come under criticism in Congress, where the Republican-controlled House has passed a bill to strip the EPA from using the Clean Air Act to regulate global warming gases. The measure failed in the Senate, but a majority there indicated they would back reining in EPA in some way.

Justice Ruth Bader Ginsburg, writing for the court, said the Clean Air Act gives the EPA authority to regulate carbon-dioxide emissions from power plants.

The landmark environmental law leaves no room for what Ginsburg described as a parallel track, “control of greenhouse gas emissions by federal judges.”

On the other hand, Ginsburg said, that the states and conservation groups can go to federal court under the Clean Air Act if they object to EPA’s eventual decision.