Four fire engines and around 25 firefighters were called to a fire at a coffee shop on Eltham High Street.
Part of a staff room on the ground floor was damaged by the fire. There were no reports of any injuries.
The fire was discovered by employees who smelt burning and saw light smoke. Staff and customers promptly left the building and called 999.
The Brigade was called at 1254 and the fire was under control by 1421. Fire crews from Eltham, Lee Green, Sidcup and Lewisham fire stations were at the scene.
The Brigade's Fire Investigators believe the fire was accidental and caused by the failure of a capacitor installed in a light fitting.