Eight fire engines and around 60 firefighters tackled a flat fire on Whitehorse Road in Croydon.
Two flats and two balconies on the first and second floors of the building were destroyed by fire. Most of a third-floor flat and balcony were also damaged by the blaze. There were no reports of any injuries.
The Brigade's 999 Control Officers took over 40 calls about the fire.
Assistant Commissioner for Fire Safety, Charlie Pugsley, said: “We are offering our support to the local community following this devastating fire and will be on scene throughout the week engaging with residents regarding any concerns they may have.
“The fire started on a balcony with timber decking and spread to neighbouring flats. The fire spread due to combustible items on the balcony, combined with the timber decking, and not due to flammable cladding materials.
“Around 80 people were already out of the building before the Brigade arrived at the scene.
"It is the responsibility of the responsible person to ensure they have a Fire Risk Assessment (FRA) and review it regularly. This includes whenever significant changes have been made to the building that would impact on the FRA or on the evacuation policy."
The Brigade was called at 1703 and the fire was under control by 1845. Fire crews from Croydon, Woodside, Wallington and surrounding fire stations attended the scene.
The fire is believed to be accidental and caused by an unattended barbecue.