Shropshire Fire and Rescue Service has said investigations by specialist officers concluded the cause of the blaze was deliberate.
Officers have condemned the action, saying deliberate fire take up valuable time and resources.
Six fire crews went to the blaze just before 12.30am on Wednesday, arriving to find flames leaping out of the roof.
The service said firefighters worked in difficult conditions to contain the fire to one unit and ensure it didn't spread to other buildings.
Residents were urged to keep windows and doors closed because of the thick pall of smoke and damage to electricity lines saw half of the town lose power while engineers fixed the problem.
While crews continued to damp down the building throughout the day a fire investigation team and forensic officers from West Mercia Police searched the building.
Fire Investigation Officer Scott Bishop said: “We carried out a full and thorough joint investigation with our partners to determine the cause of the fire.
“Unfortunately, we concluded the blaze was started deliberately. Deliberate fires take up valuable time and resources which could be used elsewhere to help others.
“We will be working closely with West Mercia police who will now be investigating the incident.”