Twelve fire engines and around 80 firefighters tackled a fire at a flat above a shop on High Road in Chadwell Heath.
Two flats on the first floor were destroyed by fire and part of the roof of the property was also damaged. Around 19 people left the building before the Brigade arrived. Three people, including a child, were taken to hospital.
A London Fire Brigade spokesperson said: “There were smoke alarms in this property, but they failed to alert the residents to the fire.
"We urge people to have working smoke alarms on every level of your home and to test them regularly. Smoke alarms will give the earliest possible warning if a fire breaks out in your home.
“As a minimum you should have smoke alarms on every floor - in the hallways and the rooms you use the most, plus a heat alarm in the kitchen and the bathroom."
The Brigade's Control Officers took the first call at 0406 and mobilised crews from Ilford, Dagenham, Romford, Hainault and surrounding fire stations to the scene. The fire was under control by 0729.
The fire is believed to have been accidental but the exact cause has been recorded as undetermined.