A judge overturned a settlement allowing major outdoor advertising companies to convert more than 800 billboards in Los Angeles into digital displays, but declined to revoke permits already granted for about 100 conversions.

Superior Court Judge Terry Green said in his decision issued Thursday that the city must withdraw from its 2006 agreement with CBS Corp.'s Outdoor division and Clear Channel Outdoor Holdings Inc.

The companies had settled a lawsuit challenging the city's billboard ban in place at the time.

Summit Media, a smaller billboard company, had sued the city over that settlement, arguing that it selectively exempted the large firms from zoning laws.

Green agreed in his ruling that the settlement was illegal, but said that the city should decide whether to revoke permits that it already issued for digital conversions.

CBS Outdoor spokeswoman Shannon Jacobs said in a statement that the company believes the agreement is valid and that it plans to appeal the ruling.

Summit attorney Anthony P. Alden had no immediate comment. Messages left with Clear Channel Outdoor and the city attorney's office were not immediately returned.