A fire at the site of a former high school is believed to have been started deliberately, fire bosses said.
A blaze broke out at the empty Habergham High School building in Kiddrow Lane, Burnley at about 20:45 GMT on Sunday. No-one was injured.
Seven fire engines attended the fire at the two-storey building, Lancashire Fire and Rescue said.
An investigation is under way but the fire service said it was being treated as a suspected arson.
The derelict school closed permanently in 2006 after merging with Ivy Bank to form Hameldon Community College.