Four fire engines and around 25 firefighters were called to a flat fire on Devonshire Avenue in Sutton.
Part of a four-roomed flat on the second floor of the building was damaged by fire. Two adults and two children left the building before the Brigade arrived. There were no reports of any injuries.
Station Officer Nigel McLachlan, who was at the scene, said: “On arrival crews saw smoke coming from the second floor of the building and gained access quickly to discover a well-developed fire in one of the bedrooms of the flat.
“A family were already out of the property when firefighters arrived, and crews worked swiftly to bring the fire under control.
“There were no working smoke alarms fitted inside the flat. 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 0239 and the fire was under control by 0358. Fire crews from Sutton, Wallington and Mitcham fire stations attended the scene.
The fire is believed to have been accidental and involved an extension lead.