Firefighters rescued a woman from a fire at a ground floor flat on Penwith Road in Earlsfield.
The woman was suffering from smoke inhalation and minor burns. She was taken to hospital by London Ambulance Service crews.
The mid-terraced property was badly damaged by fire, heat and smoke, and is now believed to be uninhabitable.
The fire was discovered by a neighbour who was alerted by a smoke alarm sounding and saw smoke issuing from the letterbox of the flat. They forced open the front door and attempted to reach the woman but were unable to do so, due to the intense heat and smoke. They dialled 999.
The Brigade's fire investigation team believe the cause of the fire was accidental. An electric blanket was folded and plugged in to the mains, which may have caused the heating filaments inside it to overheat and catch fire.
A London Fire Brigade spokesperson said: "As the weather turns colder, more people will be using electric heaters and electric blankets at home.
"Always store electric blankets flat, rolled up, or loosely folded to prevent damaging the internal wiring. Unplug blankets before you get into bed, unless your blanket has a thermostat control for safe all-night use. Always unplug it when going out or when it is otherwise not in use."
The spokesperson added: "The woman's neighbour acted quickly and selflessly. We urge anyone who hears a smoke alarm going off in a neighbouring house or flat to dial 999 immediately and report it."
The Brigade was called at 0444 and the fire was under control by 0555. Three fire engines and around 15 firefighters from Wandsworth and Tooting fire stations attended the scene.