The fire at the McKechnie Jess meat processing plant on Port Glasgow Road, Greenock was reported just after 3.20am on Wednesday and involved 30 firefighters who were still at the scene this afternoon.
It prompted the closure of the A8 while crews tackled the blaze.
Local diversions via the B768 & B788 under convoy were put in place during the day.
Traffic Scotland told commuters: “The A8 is closed near Ladyburn in Greenock due to a building fire and access for emergency services. A local diversion is in place and HGVs are being stacked and let through at intervals. Road users are advised to follow the local diversion and expect longer then normal journey times.
A spokeswoman for the Scottish Fire & Rescue service said: “Around 30 firefighters are currently tackling a significant fire at a Greenock factory.
“It was 3.22am when the alarm was raised on Wednesday. There are no casualties.”