Four fire engines and around 25 firefighters were called to a maisonette fire on North Street in Plaistow.
Half of a four-roomed maisonette on the first floor of the building was damaged by fire. Three men and a woman left the building before the Brigade arrived. There were no reports of any injuries.
The fire is believed to have been accidental and caused by 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 1735 and the fire was under control by 1842. Fire crews from Plaistow, Poplar, Stratford and Leytonstone fire stations attended the scene.