Melbourne factory blaze deemed suspicious

A factory fire in Melbourne that took 50 firefighters two hours to bring under control has been deemed suspicious.

MFB and CFA crews battled the Hoppers Crossing blaze early on Thursday but had to withdraw and fight the flames from outside over fears the concrete walls could collapse.

The blaze broke out in a complex of four factories made of tilt slabs in Graham Street just after midnight.

MFB investigators have determined the fire is suspicious and have referred the incident to Victoria Police.