Four fire engines and 21 firefighters and officers were called to a fire in an outbuilding in Chestnut Avenue, Northwood.
The single storey building was destroyed by the fire. There was also a small fire in a bin and a small rubbish fire which were both extinguished.
A cylinder was removed from the scene and cooled to an ambient temperature.
The Brigade was called at 0131 and the fire was under control at 0300. Fire crews from Harrow and Ruislip fire stations and colleagues from Hertfordshire Fire and Rescue Service attended the scene.
The cause of the fire is under investigation by the Brigade and the Metropolitan Police Service.