Four fire engines and around 25 firefighters were called to a flat fire on St James's Crescent in Brixton.
Part of a roof on the fifth floor of the building was damaged by fire. There were no reports of any injuries.
The Brigade was called at 1005 and the fire was under control by 1055. Fire crews from Brixton, Clapham and Lambeth fire stations attended the scene.
The Brigade's Fire Investigators believe the fire was accidental and caused by an electrical inverter for solar panels.