A linen factory has gone up in flames in Melbourne's north, leaving a $1 million dollar damage bill.

The Country Fire Authority said 55 fire crews spent 40 minutes controlling the factory fire - which saw part of the roof cave in - in Thomastown, 17km north of central Melbourne, overnight Thursday.

Fire crews are investigating the cause, but say early signs suggest the blaze was not suspicious.

Fire investigators later said self-combustion sparked the blaze, caused by a failure to allow linen in the factory to cool sufficiently as it was being folded and stacked, the MFB said.

 

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