A fire at the former Kirklees College building was “deliberately started”, officials say.
The blaze broke out on the top floor of the derelict building in New North Road, Huddersfield, on Thursday evening.
At one point, 20% of the floor was engulfed in flames.
The fire service said they were called out after numerous members of the public reported seeing smoke billowing from the building , which is due for demolition.
Five appliances were sent to the site at 7.45pm on Thursday, before police arrived to close the surrounding roads.
Watch commander Andy Rose, from Huddersfield Fire Station, said the fire was thought to be deliberately started as all electric mains in the building are switched off.
He said: “The fire was confined to one classroom on the top floor.
“We used two aerial platforms with a crew member hoisted up with a pump to dampen the fire.
“We then sent a team up on to the top floor, with precautions, to extinguish the fire.”
He added: “This fire was started deliberately, there is no other possible cause as all the mains in the building have long been switched off.
“We are not sending a fire scene investigator due to the derelict nature of the building, but police are investigating.”
The building has now been boarded up with measures taken to ensure it is securely sealed off to the public.
A spokesperson for West Yorkshire Police said: “We are investigating this as arson.
“If anyone witnessed anything suspicious or has any information, they are urged to call Huddersfield CID via 101, quoting reference 13160533046 of October 27.”
The blaze is the fourth at the building this year. The site was last visited by firefighters in June, as well as in April and February earlier this year.
