Eight fire engines and around 60 firefighters were called to a fire at a shop with a flat above on Station Parade in Barking.
Half of the first floor flat was destroyed by fire.
No injuries are reported.
Station Commander Paul Green was at the scene, he said: "Crews worked extremely hard in hot arduous conditions to extinguish the fire."
Fire investigators believe the fire was accidental and caused by the careless disposal of smoking materials.
A London Fire Brigade spokesperson said: "This is a timely reminder for residents to ensure they have working smoke alarms in the home. They give the earliest possible warning to fire, providing time to escape.
"You should have smoke alarms in every room where a fire could start, plus a heat alarm in the kitchen.
"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.
"It's dangerous to throw cigarettes out of windows or off balconies as they can easily catch light to items below and start a serious fire.
“By not disposing of your cigarettes properly, you risk causing a fire which could not only destroy your home, but also cost you your life.”
The Brigade was called at 1909 and the fire was under control by 2007. Fire crews from Stratford, Poplar, Barking, Plaistow, Dagenham, Barking, Millwall and East Greenwich fire stations were in attendance.