Firefighters were called to a fire below a sandwich shop on Goodge Street in Marylebone yesterday afternoon.
The fire was located in the basement, which was slightly damaged by the fire.
Four people left the building before the Brigade arrived and no one was injured.
It is believed the fire was caused by a discarded cigarette dropped by a member of public.
14 firefighters and officers from Euston and Soho Fire Stations attended the incident.
The Brigade was called at 1627 and the fire was brought under control at 1656.