"Because greenhouse gases fit well within the Clean Air Act's capacious definition of 'air pollutant' we hold that EPA has the statutory authority to regulate the emission of such gases from new motor vehicles," the court ruled.

Led by Massachusetts, a dozen states along with several US cities and environmental groups went to the courts to determine whether the agency had the authority to regulate greenhouse gases such as carbon dioxide emissions.

"The harms associated with climate change are serious and well recognized," said judge John Paul Stevens in the sharply divided ruling with five votes in favor to four against.

"EPA's steadfast refusal to regulate greenhouse gas emissions presents a risk of harm to Massachusetts that is both 'actual' and 'imminent.'"

The Bush administration has fiercely opposed any imposition of binding emissions limits on the nation's industry and has refused to ratify the Kyoto Protocol on reducing greenhouse gas blamed for global warming.

Environmentalists have alleged that since Bush came to office in 2001 his administration has ignored and tried to hide looming evidence of global warming and the key role of human activity in climate change.

At a hearing in November, Massachusetts argued that it risked losing more than 4.5 meters (15 feet) of land all along its coastline if the sea level should rise 30 centimeters (one foot).

But the Bush administration backed by nine states and several auto manufacturers urged the court not to intervene, arguing that if the situation was so dire it could not be solved by a simple legal decision.

It further argued that reducing emissions from new US motor vehicles would have only a minor effect on global climate change.

"While it may be true that regulating motor-vehicle emissions will not by itself reverse global warming, it by no means follows that we lack jurisdiction to decide whether EPA has a duty to take steps to slow or reduce it," the court ruled.

White House spokeswoman Dana Perino said the administration and EPA would have to "analyze" the decision and denied ever contesting that greenhouse gases contribute to global warming.

Environmental campaigners, who have been fighting for greater regulations in a nation which accounts for a quarter of all global greenhouse gas emissions, hailed Monday's ruling, as did a handful of politicians.

"This is a total repudiation of the refusal of the Bush administration to use the authority he has to meet the challenge posed by global warming," said Josh Dorner, spokesman for the Sierra Club environmental group.

It also "sends a clear signal to the market that the future lies not in dirty, outdated technology of yesterday, but in clean energy solutions of tomorrow like wind, solar," he added.

Bush is strongly opposed to Kyoto's approach of binding cuts, also called caps, in emissions and has instead promoted voluntary action, backed by some incentives for cleaner energy sources and gains in energy efficiency.

California Governor Arnold Schwarzenegger, whose state is among several to have already acted independently to reduce greenhouse gas emissions, said he was "very encouraged" by the ruling.

Senator and presidential candidate Hillary Clinton said the decision "rejects the Bush administration's 'do nothing' approach to the problem," while fellow candidate, Senator Barack Obama, said that "after six years of inaction and denial our government must take bold action to save our planet."

While the court's decision is unlikely to change US policy, it has ramifications on several other ongoing issues, such as the agency's refusal to regulate emissions from electricity plants which produce some 40 percent of US carbon dioxide emissions. Motor vehicles are responsible for just 20 percent.

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