Crew Fitness and Performance in Braybrook was destroyed by fire on Tuesday morning.
Multiple calls were received at 3.19am for a car fire inside a factory on Cranwell Street.
FRV appliances arrived on scene in six minutes, but the fire had already taken hold inside the two storey building.
Firefighting crews worked quickly to bring the fire under control at 3.38am before it could spread inside the complex and to adjoining buildings.
The gymnasium has sustained severe damage.
The cause has been deemed suspicious and will be investigated by police.
