The building, in Newhaven, was evacuated and no injuries have been reported so far.
Twelve fire engines attended the scene, as well as two water carriers. The fire has not been put out yet and the cause is unknown.
East Sussex Fire and Rescue was called at 9.32am on Saturday.
Big fire in Newhaven at the moment. pic.twitter.com/TmYkUfElIO
— Joel Jenvey 🇪🇺 (@jdjenvey)
The service said in a statement: "An industrial building at Beach Road is alight. More than 80 999 calls have been made to us.
"The building has been evacuated. As of 10.30 there are 12 fire engines at the scene along with two water carriers.
There are no reports of injuries. Please avoid the area and keep doors and windows closed to avoid the smoke."