Five fire engines and around 35 firefighters were called to a flat fire on Shoot-Up Hill in Kilburn.
Part of a four-roomed flat on the second floor of the building was damaged by fire. There were no reports of any injuries.
The fire was discovered by a neighbour who smelt burning and upon investigation saw smoke coming from a window.
The Brigade’s Fire Investigators believe the fire was 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 your home, but also cost you your life.”
The Brigade was called at 1359 and the fire was under control by 1439. Fire crews from West Hampstead, Willesden, Paddington, Hendon and Kentish Town fire stations attended the scene.