Arsonists broke into a mill complex and set fire to a building, causing part of the roof to collapse.
Firefighters from Wigan and Hindley stations were called to Eckersley Mills, on Swan Meadow Road, Wigan, after the blaze was started just after 9.30pm on Thursday.
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Trespassers had broken into a single-storey annex workshop at the rear of the complex, vandalised it and started a fire.
Crews spent just over an hour tackling the blaze, which caused part of the roof to collapse and holes in the floor.
The owners of the disused building attended and fencing was being erected to prevent further incidents.
Glyn McGann, watch manager at Wigan fire station, said: "Trespassing, breaking into property and setting fire to it is arson and endangering life.
"It also ties up fire service time. It puts the whole community of Wigan and Hindley in jeopardy because if something happens, they are waiting for help from elsewhere. There are three fire appliances and we were tied up with this.
"It's a waste of resources and costs thousands of pounds."