A factory has been destroyed by a fire at Thomastown in Melbourne's north.

Emergency crews were called to Temple Drive around midnight to find the building well alight.

The blaze also threatened adjacent factories.

The Metropolitan Fire Brigade's Mark O'Conner says the blaze was fuelled by large bails of cloth.

"It's caused some damage to the roof which has started to sag, so we're acutely aware of fire fighters safety," he said.

"The premises will be out of action for a considerable amount of time."

The blaze has left a damage bill of more than a million dollars.

Investigations have revealed the cause of the fire was spontaneous combustion of the linen.

The MFB says the linen was not allowed to cool down properly before being folded and stacked.