WASHINGTON, Jan 23 (Reuters) - The U.S. Environmental Protection Agency figured it would lose a lawsuit filed by California if the agency turned down the state's request to toughen vehicle emissions standards to fight global warming, according to documents released on Wednesday.

The suit the documents envisioned was filed on Jan. 2, after the agency rejected a petition by California seeking a waiver from federal law to impose more stringent standards for greenhouse gas emissions from cars, light trucks and sport utility vehicles. Fifteen other states were poised to follow California's lead, if the waiver had been approved.

In EPA briefing documents released by the U.S. Senate Environment and Public Works Committee, the agency listed the likely impact of granting or denying a waiver.

The documents accurately predicted what would happen next if the EPA rejected the waiver: The agency would face an "almost certain lawsuit by California." The EPA also said it was "likely to lose (the) suit." The federal appeals court in San Francisco has yet to issue a ruling.

If the EPA had granted the waiver, the documents said the agency would likely face a legal challenge from automakers. But the agency calculated it would almost certainly "win such a suit."

The documents concluded that granting the waiver would "likely allow (California) standards to go into effect" and that this would be "generally consistent" with federal rules on greenhouse gases.

AUTO FIRMS

Auto companies, especially struggling U.S.-based manufacturers, fiercely opposed the attempt by California to impose new emissions regulations, which would sharply increase mileage requirements for their vehicles in that state and any others that adopted the change.

Automakers and their allies in Congress believe the California standard would be too tough to meet. The measure was included in a 2002 state law that would force automakers beginning next year to cut tailpipe emissions by 30 percent over eight years. That translates into average fuel efficiency of more than 40 miles per gallon.

Industry, backed by the EPA, favors new federal fuel efficiency regulations that will require fleets to average 35 miles per gallon by 2020 -- a 40 percent increase over today's level but less stringent than California's plan.

The Senate committee released the documents the day before EPA chief Stephen Johnson was scheduled to testify about the decision. The documents were obtained from committee staff, not the EPA, which earlier had offered heavily redacted versions, with large portions whited out.

Sen. Barbara Boxer, a California Democrat who chairs the committee, was outraged: "This unbelievable brush-off by the EPA only makes me more determined to get to the truth."

Johnson was given a wide array of information on which to base his decision and he stands by it, said agency spokesman Jonathan Shradar.

Shradar confirmed the documents released by the committee were those created by the EPA.

California has for years been permitted to set its own targets on pollutants and has gotten EPA waivers in other cases. This is the first bid to limit carbon emissions from autos. (Additional reporting by John Crawley, editing by Todd Eastham)