The Environmental Protection Agency announced plans Monday to ban major new sources of air pollution in the nation's most populous urban area, greater Los Angeles.

EPA Administrator Lee M. Thomas said similar action is planned against at least three other cities by the end of this year unless Congress revises the Clean Air Act.California officials have said they expect little effect from the Los Angeles ban, which goes into effect Tuesday, because it covers only large new plants that no one is planning to build in the area, such as oil refineries.

A stopgap congressional ban on agency sanctions expires at midnight Tuesday, eight months after it was imposed to buy time for Congress to produce changes in the law.