Fire crews from across district were called to a blaze at an old mill building in Sunbridge Road in Bradford city centre at 12.30pm today.
Flames shot from the roof as a crowd of more than 100 people stood watching.
More than ten fire crews were involved in tackling the blaze.
It is believed that the mill, near the junction with Vincent Street, was being converted into luxury flats and offices. Construction workers had to be evacuated.
Fire crews used an aerial platform to try to tackle the blaze from above and surrounding roads have been cordoned off.
A thick plume of black smoke continued to billow into the air at 1.30pm and burning timbers could be seen following the collapse of the roof of the building.
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