Firefighters were called to a fire at a shop with flats above used as a suspected cannabis factory on Westow Hill in Crystal Palace.
Part of the lower-ground floor of the building was damaged by fire. Two people left the flats above before the Brigade arrived. There were no reports of any injuries.
The alarm was raised by a resident in a flat above who could smell smoke coming from a shop on the ground floor.
A London Fire Brigade spokesperson said: “Firefighters arrived to find a small fire within an extraction system in the basement.
“Cannabis factory fires can be particularly dangerous because of the way criminals use unsafe wiring to illegally obtain electricity in order to grow the plants.
"They're often in top floors or lofts which means when a fire takes hold it spreads, destroying roofs and damaging neighbouring buildings.
"Firefighters can also be exposed to risks when dealing with the fires because of bad wiring and traps at some of the properties.
"It's important that people know the key signs and inform the police so that they can act swiftly to prevent these dangerous fires from happening."
The Brigade was called at 1115 and the fire was under control by 1257. Two fire engines and around 10 firefighters from West Norwood and Norbury fire stations attended the scene.
The Brigade’s Fire Investigators believe the fire was caused by the failure of an extractor fan in the ventilation system.