Four fire engines and around 25 firefighters tackled a fire on Cator Street in Camberwell.
Part of a fifth-floor flat in a six-storey residential building was damaged by fire. One man was treated by London Ambulance Service crews for smoke inhalation and taken to hospital.
The fire is being treated as accidental and is believed to have been caused by the unsafe disposal of smoking materials - a cigarette.
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 1555 and the fire was brought under control by 1632. Crews from Old Kent Road, Lambeth, Brixton and Greenwich fire stations attended the scene.
Firefighters' smoking safety tips
· Use proper ashtrays, which can’t tip over and stub cigarettes out properly.
· Don’t leave lit pipes or cigarettes unattended.
· It's safer to smoke outside, but make sure cigarettes are put right out and disposed of properly.
· Don't balance cigars or cigarettes on the edge of an ashtray, or anything else – they can tip and fall as they burn away and cause a fire.
· Take extra care when you’re tired, taking prescription drugs or if you’ve been drinking alcohol.
· Always empty ashtrays carefully. Make sure smoking materials are out, cold and preferably wet them before throwing into a bin – never use a wastepaper basket.