Environmental campaigners in California have attacked carmakers after they mounted a legal challenge to the state's emissions caps.

California has introduced tough new legislation placing limits on CO2 and other greenhouse gas emissions.

The limits, which are due to come into force between 2009 and 2012, were challenged in the courts this week by a group of automobile dealers in California's Central Valley and the Alliance of Automobile Manufacturers.

In response, the California Clean Cars Campaign accused the carmakers of attacking the state's right to "protect its citizens from auto air pollution".

The campaign group warned that the emissions caps would be a vital step in reducing smog and tackling related illnesses such as asthma.

"Cars and light trucks are the largest source of global warming pollution in California, accounting for 40 per cent of all greenhouse gas emissions in the state," the group said.

The Alliance of Automobile Manufacturers argues that individual states should not impose stricter tailpipe emission caps than those set out under existing federal regulations.