Six fire engines and around 40 firefighters were called a fire at a disused bank over five floors on Great Portland Street.
A small area of an underground electricity substation in the basement of the building was damaged by fire.
The Brigade’s fire investigators found that the cause was accidental. A fuse in the basement of the building had malfunctioned.
Fire crews successfully contained the fire to the basement.
The Brigade was called at 0534 and the fire was under control by 0749. Fire crews from Euston, Soho, Paddington, Kentish Town and Islington fire stations attended the scene.