Four fire engines and around 25 firefighters were called to a fire at a block of maisonettes on Wandsworth Road in Lambeth.
A maisonette on the top floor of a six-storey block was destroyed by the blaze. One man left the affected property before the Brigade arrived and was treated at the scene for smoke inhalation by London Ambulance Service crews.
A further four people self evacuated from surrounding properties.
The Brigade's 999 Control Officers have taken 12 calls to the blaze.
The Brigade's Fire Investigators believe the fire was caused by the unsafe handling of smoking materials.
A London Fire Brigade spokesperson said: "Fires caused by smoking materials including cigarettes, roll-ups, cigars and pipes, result in more deaths than any other type of fire. Stub cigarettes out properly and always dispose of them carefully.
"Make sure smoking materials are cold before emptying ashtrays. Preferably wet stubs before throwing them into a bin.”
The Brigade was called at 1214 and the fire was under control by 1306. Fire crews from Brixton and Clapham fire stations were at the scene.