Five non-profit groups have filed a lawsuit challenging the 19 December decision by the US Environmental Protection Agency (EPA) to deny California its request to implement its landmark law limiting global warming pollution from new vehicles.

The state of California has also filed its own legal challenge to EPA's decision in the same court. It was expected that 15 states would file a motion to intervene in support of California. Pennsylvania has confirmed it is joining the multi-state lawsuit.

The non-profit petitioners - the Conservation Law Foundation, Environmental Defence, International Centre for Technology Assessment, Natural Resources Defence Council (NRDC) and the Sierra Club - filed the suit in San Francisco.

"While global warming marches onward, EPA continues to drag its feet," said Environmental Defence lawyer Jim Tripp. "The agency's decision defies the law, the science and the will of states representing nearly half of the US population."

At stake, the group claims, are historic California standards to lower global warming pollution from passenger cars and trucks. The California standards are scheduled to take effect in model year 2009 and secure a 30% fleet wide reduction by 2016. The state programme would be the first binding one in the US to strictly limit global warming pollution, rather than simply mandating fuel economy.

The EPA has consistently granted more than 50 such requests from California over the past 40 years for waivers under the Clean Air Act. The federal law broadly guarantees California's right to adopt its own motor vehicle emission standards so long as they are more protective than the federal emission standards. There are no federal greenhouse gas emission standards in place for any pollution source.

Sixteen other states have adopted or have committed to adopt the California standards. Collectively, California and the other states account for nearly one-half of the US population and about 45% of all new vehicle sales nationwide.

"The administrator's denial of California's request relies on a flawed argument that the federal courts already have rejected," added Tripp. "We've won before in the federal courts, so we expect to win again this time too."

"California has long played a pioneering role in clean air efforts," said Karen Douglas, director of the California Climate Initiative of Environmental Defence. "We can and must reduce global warming pollution from automobiles."

California's binding statewide cap on global warming pollution calls for cutting pollution to 1990 levels by 2020 as called for in AB 32, the Global Warming Solutions Act. The cap will drive further cuts in greenhouse gas emissions from motor vehicles between 2016 and 2020, so several phases of reductions, advancing new generation of technologies, will be implemented in California.