Firefighters tackled a fire at a flat on Ley Street in Ilford.
A boiler cupboard on the ground floor of a residential block was damaged by fire. There were no reports of any injuries.
A London Fire Brigade spokesperson said: “The fire was discovered when a smoke alarm in the flat sounded. There was smoke in the hallway and a resident opened the door to a cupboard containing an immersion heater and found it filled with smoke. They closed the cupboard door and left the property along with four children.
“This incident is another reminder about the importance of having smoke alarms fitted in your home.
"You should have working smoke alarms in every room where a fire can start, plus a heat alarm in the kitchen. They should be tested regularly."
The Brigade was called at 0530 and the fire was under control by 0615. Six fire engines and around 40 firefighters from Ilford, Stratford, Barking and East Ham fire stations attended the scene.
The fire is believed to have started when the water supply to the immersion heater was isolated, leaving the cylinder dry. The air in the cylinder was then heated, causing surrounding foam insulation to melt and burn.