Four fire engines and around 25 firefighters tackled a flat fire on Billington Road in New Cross Gate.
Part of a two-roomed flat on the ground floor of a converted terraced house was damaged by fire. Two people left the property before the Brigade arrived. There were no injuries reported.
The fire is believed to have been accidental and caused by the unsafe disposal of smoking materials.
A London Fire Brigade spokesperson said: "We'd rather you didn't smoke at all, but if you are a smoker, it's absolutely vital you ensure your cigarette is completely out when you've finished smoking it.
"If you don't, you risk causing a fire which could not only destroy properties but also cost you your life."
The Brigade was called at 1446 and the fire was under control at 1538. Fire crews from New Cross, Peckham, Greenwich and Deptford fire stations attended the scene.