California is to lose its authority to set higher fuel efficiency standards for cars, Donald Trump announced yesterday.
In a tweet, the US president insisted his move to revoke the state's authority to set higher standards than the US norm would result in less expensive and safer cars.
And he insisted new cars would be cleaner, even as they burn more petrol than they would have under Obama-era fuel efficiency standards.
However, his claims have been challenged by both US car makers and climate change activists.
The car makers contend that without a substantial increase in efficiency, vehicles will be less competitive globally, which could mean job losses.
Mr Trump's move comes after the Justice Department opened a competition investigation into a deal between California and four car makers for tougher pollution and related mileage requirements than those sought by the president.
He has also sought to relax Obama-era federal mileage standards nationwide, weakening a key effort by his Democratic predecessor to slow climate change.
Environmental groups and California officials have pledged legal action to stop the rollback.
Environmental Protection Agency administrator Andrew Wheeler told the National Automobile Dealers Association this week that the goal is to establish one nationwide set of fuel-economy standards.
"We embrace federalism and the role of the states, but federalism does not mean that one state can dictate standards for the nation," Mr Wheeler said.
He added that higher fuel economy standards would hurt consumers by increasing the average price of new cars and requiring firms to produce more electric vehicles.
Word of the announcement came as Mr Trump travelled to California for a trip that included fundraising events.
California's authority to set tougher emissions standards goes back to a waiver issued by Congress during the passage of the Clean Air Act in 1970.
The state has long pushed for fuel-efficient passenger vehicles that emit less pollution. A dozen states and the District of Columbia follow its fuel economy standards.
California attorney general Xavier Becerra said Mr Trump's action will hurt US car makers and families.
He added that California would fight the administration in federal court.