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The Trump administration has drafted a new set of regulations on planet-warming emissions from cars and light trucks that would dramatically weaken Obama-era standards. The proposal, if implemented, would also set up a legal clash between the federal government and California by challenging the state’s authority to set its own, stricter, air pollution rules.

Details of the proposal, which is being jointly drafted by the Environmental Protection Agency and the Transportation Department and is expected to be sent to the White House for approval in coming days, were described to The New York Times by a federal official who had seen them but was not authorized to discuss the matter.

The proposal follows an announcement this month by the E.P.A. administrator, Scott Pruitt, that the Trump administration intended to weaken the stringent vehicle fuel economy standards set by the Obama administration that aimed to roughly double the average fuel economy of new cars, S.U.V.s and light trucks by 2025.

The E.P.A. declined to comment on the emissions proposal, which could still change before being made public.

In the draft, the agencies lay out eight different options for revising the Obama-era standards. The preferred course of action would freeze fuel-economy standards at 2020 levels for both cars and light trucks, greatly slowing progress in reducing auto emissions.

The proposal also challenges California’s authority to impose its own vehicle standards.

Currently, California has a waiver under the Clean Air Act to impose its own, stricter, air pollution regulations on cars and trucks to deal with problems like smog. But the administration’s draft proposal argues that California cannot use this waiver to set standards on greenhouse gas emissions from vehicles because that would be tantamount to regulating fuel economy, which states are forbidden from doing under a 1975 law.

The auto industry has previously tried to challenge California’s greenhouse gas standards for vehicles on these grounds, but federal courts have so far rejected their arguments. Legal experts have said that, as long as California is regulating the pollutants that come out of tailpipes and not directly determining fuel economy standards, the state is on solid legal ground.

The Trump administration has also signaled that it would consider rescinding California’s waiver altogether, although the draft proposal does not mention this. On Thursday, Mr. Pruitt told Congress that the agency was still in “active discussions” with California and had no plans at the present to revoke the waiver.

In recent months, automakers have become increasingly nervous about the Trump administration’s collision course with California. A group of automakers has requested a direct meeting with President Trump to urge the administration to avert a legal clash with California, which could plunge the auto industry into regulatory chaos, according to two people with knowledge of the automakers’ plans. A spokeswoman for the Alliance of Automobile Manufacturers was not immediately available for comment.

For its part, California has declared it will stick with the stricter, Obama-era regulations, a decision that could effectively split the United States into two auto markets and set up a messy legal battle.

Stanley Young, a spokesman for the state’s clean air regulator, the California Air Resources Board, said the agency was not aware of any official proposal. But if true, “this would harm people’s health, boost greenhouse gas pollution and force drivers to pay more money at the pump for years.”

“It would also severely disrupt the U.S. auto industry, compromising its ability to succeed in a highly competitive global market that increasingly values innovative and efficient technologies,” Mr. Young said.

A rollback of the rules, which were designed to reduce emissions of greenhouse gases, would blunt one of the single biggest steps any government has taken to tackle climate change.

Adopted in 2012, the Obama-era standards would have required new cars and trucks to average more than 50 miles per gallon by 2025 if automakers complied solely by improving the fuel economy of their engines. (Because automakers can get credit for actions like using less-polluting refrigerants in air-conditioning units, the actual fuel economy of new vehicles is expected to be lower.)

But under the draft proposal’s preferred outcome, fuel economy standards would be frozen after 2020, keeping the fuel economy target closer to 40 miles a gallon through 2025.

Other possible approaches in the draft proposal involve progressively increasing fuel economy by between 0.5 percent and 2 percent a year for cars, and from 0.5 percent to 3 percent for light trucks.

The E.P.A. and Transportation Department are expected to send their proposal to the White House’s Office of Management and Budget in the coming days for review. Once the proposal is reviewed and published in the federal register, it will have to undergo a public comment period and may see further changes before being finalized.

Because automakers already have the technology in place for their 2020 models, freezing those standards would be tantamount to ceasing to regulate fuel economy improvements altogether, said Ann E. Carlson, a professor of environmental law at the University of California, Los Angeles.

“This is essentially saying to automakers: Keep doing what you’ve already been doing,” she said. “It’s like saying: We are not going to regulate you anymore. You’re already geared up to meet the standards and we’re finished.”

Hiroko Tabuchi reported from New York, and Brad Plumer and Coral Davenport from Washington.

A version of this article appears in print on April 27, 2018, on Page A15 of the New York edition with the headline: Plan to Loosen Emission Limits Disputes California’s Right to Set Its Own. Order Reprints | Today’s Paper | Subscribe