Melbourne factory fire deemed suspicious after petrol can found near entrance

A ferocious fire has engulfed a Melbourne factory, bringing down the roof.

It took 30 firefighters an hour to bring the fire at the Campbellfield foam factory under control after arriving around 5am today.

It is believed the fire was burning for some time before fire crews arrived.

A can of petrol was found near the front door of the building, a policeman at the scene told 9NEWS.

The Metropolitan Fire Brigade said the fire has been declared suspicious.

Investigations into the blaze are underway.