WASHINGTON -- Lawyers for environmental groups said today they plan to join California next week in suing the EPA to allow states to enforce greenhouse gas emission rules for cars and trucks.

California asked the EPA in late 2005 for permission to carry out its landmark rules limiting greenhouse gases from cars and trucks. Twelve other states have adopted similar rules. Others are considering them.

The EPA has not decided on the request. California officials have said they will sue by Oct. 24 if the permission -- a waiver under the federal Clean Air Act -- is not granted.

The environmental lawyers said in a telephone news conference they know of no reasonable legal grounds for the Bush administration to deny the waiver.

They conceded that the administration, which is preparing first-ever national greenhouse gas emission rules, could contend that the proposed federal standards would do more to protect human health than the California rules.

But David Bookbinder, senior attorney for the Sierra Club, said he "would be astonished" if the administration makes such a claim.

Under California rules, effective next year, automakers would be required to reduce emissions by about 30 percent by 2016. Because the main greenhouse gas, carbon dioxide, is a byproduct of burning fuel, the industry likely would have to improve fuel efficiency by a comparable amount.

The rules have been upheld in a Vermont federal court. Automakers are appealing that decision and continue to challenge the rules in a California federal court.

The suit over the EPA waiver would be another of a growing number of uncertainties that face the auto industry as it tries to plan future products.

Besides the states' efforts and the planned Bush administration rules, Congress is edging toward approval of energy legislation that would require vehicles to average 35 mpg by 2020 -- about 40 percent higher than today.