Firefighters battled for two hours during the night to extinguish a blaze in a derelict building.
First reports suggested smoke was coming from Crawley Hospital but fire crews found it was the three-storey derelict block in Robinson Road.
Around 45 firefighters tackled the blaze early this morning.
Crews from Crawley, Horley, Horsham, Turners Hill and Haywards Heath searched the building but found no one inside.
Assistant divisional officer Brian Holden, station commander at Crawley, said the fire had been started accidentally by someone who had broken into the building.
He said: "It would appear whoever broke into the building attempted to use an old fireplace to start a fire and keep warm.
"What they did not know was that the chimney didn't go outside the building and all the embers went straight into the room above."
Mr Holden said the fire service had received a number of calls to derelict buildings in the town during the past few months.
He said: "It is a concern from our point of view, the number of derelict buildings around Crawley we are sending crews into.
"These buildings are often very dangerous and we have had several calls over a period of months."
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