Although the statement does not mention G.M., Toyota or Fiat Chrysler by name, the new policy amounts to ban on state purchases of vehicles made by those companies and a handful of others, represented by the lobbying group Global Automakers, a spokesman for Mr. Newsom confirmed on Monday.

Going forward, California’s state agencies will chiefly purchase cars from Ford, Honda, Volkswagen and BMW. In July, those companies sided with California when they struck a deal to follow California’s more stringent standards, which are close to the original Obama-era rules.

Also on Friday, California filed its second lawsuit against the Trump administration over its revocation of the state’s legal authority to set tougher pollution standards. The revocation was put forward jointly in September by the Transportation Department and the Environmental Protection Agency. The state filed suit against the Transportation Department in September, then followed up last week with its suit against the E.P.A.

It was not clear how much impact California’s new buying ban would have. California’s government has about 51,000 vehicles but only purchases 2,000 to 3,000 vehicles a year, according to data provided by the state. Of the vehicles it owns, roughly 14,000 are made by Ford, 10,000 are made by G.M., 4,000 are made by Fiat Chrysler and 1,200 are made by Toyota.

About 17 million cars and light trucks were sold across the United States last year.

The ban on new purchases does not apply to some public safety vehicles, including California Highway Patrol cars, allowing the state to continue to purchase muscular vehicles like Fiat Chrysler’s Dodge Charger, according to Mr. Newsom’s spokesman.

California’s pact with Ford, Honda, Volkswagen and BMW embarrassed the Trump administration, which assailed the move as a stunt. The Justice Department subsequently opened an antitrust inquiry into the four automakers on the grounds that their agreement with California could potentially limit consumer choice.

Since the opening days of his administration, Mr. Trump has been boasting that his rollback of vehicle pollution rules would help both automakers and car buyers. The Obama-era standard would have required automakers to build vehicles that achieve an average fuel economy of 54.5 miles per gallon by 2025, which would eliminate about six billion tons of carbon dioxide pollution, one of the chief causes of global warming, over the lifetime of the vehicles.