The fire is believed to involve a carpet and flooring business, piano and business supplies units on the Tiverton Business Park opposite B&M and next to McDonalds.
People in nearby local businesses have been evacuated.
Devon and Somerset Fire and Rescue Service said it received the first call to attend shortly after 6.30pm last night (Monday).
Fire Control said that 12 fire engines plus specialist appliances including two aerial ladder platforms and high volume pumps were sent to the scene.
It is unknown how the fire started.
At 6.40am this morning Fire Control reported a 150 metre cordon has been established for public safety and firefighters are working to prevent the fire spreading to other units.
Fire crews in attendance are Dawlish, South Molton, Danes Castle (Exeter), Bampton, Crediton, Cullompton, Tiverton, Witheridge, Middlemoor (Exeter), Urban Search and Rescue, Taunton, Bridgwater, Wellington and Wiveliscombe.