Eight fire engines and around 60 firefighters responded to a fire on Kennington Road, Lambeth.
At the fires height a flat on the 11th floor, in a residential block of 15 floors, was alight.
The Brigade was called about the incident at 2029, and crews from Lambeth, Dowgate, Soho and surrounding fire stations have been mobilised to the scene. Firefighters had the incident under control by 2141.
The cause of the fire is under investigation by the Brigade's Fire Investigation Team.