Campaigners threatened to sue the Environmental Protection Agency (EPA) on Wednesday in an effort to push Barack Obama to make good on his re-election promise to act on climate change.
The formal notice calls on the EPA administrator, Lisa Jackson, to take immediate steps to begin regulating carbon emissions from cars, planes and off-road vehicles.
It was the first attempt by campaigners since Obama's re-election to compel the agency to use its powers to act on climate change. And, once again, it puts the EPA at the centre of the political struggle over climate.
Obama is under growing pressure to take the lead on the environment, after ranking the issue among the top three challenges facing the country following his re-election.