Military crews were called to the semi-derelict Firth Park School on Barnsley Road at 2317 GMT on Sunday.

The school's main building had already suffered fire damage after two arson attacks in November 2002 during the first fire strike.

Contractors were due to start work on turning the site into a new sixth form college for north-east Sheffield in April.

Arson attack

But Sheffield Council's Education Department fears the demolition of the main building will now have to be brought forward for safety reasons.

Education spokesman Warwick Toone told BBC News Online: "There is just a shell left - the walls and roof have been extensively damaged.

"Contractors and the council's safety unit are checking the site."

Mr Toone confirmed that security had been increased at the site since the firefighters' strike began, but said: "This was another suspected arson attack."