The E.P.A.’s rule also violated the Clean Air Act because it failed to let the states submit their own plans to comply and imposed a federal plan instead, the court said. The statute leaves it to the states to decide how they will meet federal standards. The ruling could also leave the previous Clean Air Interstate Rule in place.    

Several power companies had challenged the E.P.A. rule, and were supported by more than a dozen states, mostly in the South and Midwest. North Carolina supported the rule, along with several mid-Atlantic and New England states.