Firefighters have been called to Manby Motorplex, near Louth in Lincolnshire, following an explosion and fire in a hangar at the former airfield.
The fire started at around 5.15pm and is believed to be at one of the former RAF hangars owned by Winchester Marine - which uses it to store diesel engines.
Witnesses have reported further explosions as the fire took hold and 10 fire engines have been on the scene. Firefighters expressed concerns about gas canisters inside.
No-one is reported to be injured or missing, but the public is being asked to stay away as they may hamper fire crews or put themselves at risk.
Police say there is a danger of the building collapsing and have imposed a 200m cordon.
According to the firm's website, it deals from The Old Grain Stores, in the Middle Gate area of the airfield, in diesel engines and spares, powerpacks, water pumps, generators, forklifts, cranes, and ex-MOD, NATO, and Government surplus, as well as marine engines, trailers, trucks, winches, gearboxes and bladder tanks.
The fire is not being treated as suspicious.