More than 40 firefighters were drafted in to tackle the blaze which broke out on Wyrefields, off Bracewell Avenue, at 4.25am.
Premises were evacuated amid fears of a gas explosion and a large protective cordon was placed around the scene.
There were also fears a number of gas and paraffin canisters – known to be housed within the workshops – could explode at any time.
Tony Crook, Lancashire Fire and Rescue Service's acting area manager, said: "Smoke has been pouring out the building for several hours.
"There is a real fear that it could spread to other premises.
"We have launched Operation Merlin, which means there is a potential for environmental damage, but the crews are doing all they can to isolate the fire.
The fire is thought to have started at the Malcolm McCulloch heating
business, which stores dozens of canisters containing flammable material.
The McCulloch premises have been destroyed by the blaze.
The second building affected was design and print firm SK Publications.
Police were also at the scene, but it is not believed there were any injuries.