WASHINGTON, Sept 18 (Reuters) - U.S. President Donald Trump confirmed Wednesday the Environmental Protection Agency will revoke California's waiver that allows it to require automakers to build cleaner vehicles than federal requirements demand - a move that will set off a massive legal battle.

Reuters and other outlets reported Tuesday the administration was set to formally announce the decision as early as Wednesday. The Trump administration will announce in the coming weeks a separate rule to roll back Obama-era fuel efficiency standards.

Trump urged automakers to back the action, but so far none have publicly supported revocation of California's authority. "Automakers should seize this opportunity because without this alternative to California, you will be out of business," Trump wrote on Twitter.

(Reporting by David Shepardson; Editing by Steve Orlofsky)