WASHINGTON — The U.S. Supreme Court ordered Bush administration environmental officials to reconsider their refusal to regulate greenhouse-gas emissions, giving a boost to advocates of stronger action against global warming.

The justices, voting 5–4, today said the Environmental Protection Agency didn’t follow the requirements of the Clean Air Act in 2003 when it opted not to order cuts in carbon emissions from new cars and trucks.

“EPA has offered no reasoned explanation for its refusal to decide whether greenhouse gases cause or contribute to climate change,” Justice Stevens wrote for the majority.

The ruling doesn’t necessarily mean the EPA will have to impose new regulations. Still, it adds to growing pressure on the administration, which has resisted mandatory limits on carbon emissions. The decision is a setback for General Motors Corp. and other automakers and for utilities with coal-fired plants, including American Electric Power Co. and Southern Co.

Environmentalists and 12 states, including California and Massachusetts, are seeking to force the federal agency to limit emissions from new cars and trucks. New York is leading a separate state effort to curb power-plant emissions.

The decision also helps efforts by states including California to enact their own climate-change regulations. In fighting those rules, automakers have pointed to the EPA’s conclusion that carbon dioxide isn’t an “air pollutant” subject to regulation under the Clean Air Act. The majority yesterday rejected the agency’s interpretation, saying greenhouse gases are air pollutants.

“States are fighting on multiple fronts against General Motors and this definitely is a blow for GM,” California Attorney General Jerry Brown said in an interview.