Four fire engines and around 25 firefighters were called to a fire at a block of flats on Yelverton Road in Wandsworth.
An electrical cabinet on the ground floor was damaged by fire. Three people were treated by London Ambulance Service crews for smoke inhalation and taken to hospital as a precaution.
The Brigade was called at 0844 and the fire was under control by 1035. Fire crews from Battersea, Chelsea, Clapham and Wandsworth fire stations attended the scene.
Fire investigators believe the cause of the fire was due to a failure in the mains electrical intake.