The Trump administration will move on Monday to sharply scale back fuel-efficiency rules for new cars and light trucks, gutting one of the government’s most significant efforts to reduce gasoline consumption and speed the country’s shift to electric vehicles. President Trump said on Saturday on social media that he had “just approved new Fuel Economy Standards,” adding falsely that President Joseph R. Biden Jr. had implemented an “E.V. mandate.” The transportation secretary, Sean Duffy, said in his own social media post that the move would be announced Monday. The final rule from the Transportation Department is expected to significantly weaken federal mileage standards, which have prodded automakers to increase the fuel efficiency of gasoline-powered cars and to sell more electric vehicles.

The rollback represents a major reversal of climate and energy policy, undoing years of regulatory progress aimed at curbing emissions and reducing dependence on fossil fuels. Environmental groups and consumer advocates have criticized the move, warning it could increase air pollution and raise costs for drivers over time. Automakers, however, have long argued that strict standards are difficult to meet without heavy investment in electric technology, and some have supported more flexible timelines.

The new rule is expected to set lower average fuel economy targets for model years 2027 through 2031, potentially allowing automakers to continue producing more gasoline-powered vehicles with fewer efficiency improvements. The Transportation Department has not released the full text of the rule, but officials say it will prioritize “consumer choice” and “affordability” over aggressive emissions reductions.

Legal challenges are likely, with environmental organizations already preparing lawsuits to block the rollback. The rule also faces scrutiny from Congress, where Democrats have called the move a dangerous step backward in the fight against climate change. The Biden administration had set ambitious targets for electric vehicle adoption, and this reversal could slow that momentum significantly.