Four fire engines and around 25 firefighters were called to a fire at a takeaway pizza shop with flats above on North Street in Clapham.
Part of the ground and first floors of the building and part of the external extraction system were damaged by fire. A man and a woman were treated on scene by London Ambulance Service crews after suffering from smoke inhalation.
The fire is believed to have been accidental and caused by hot embers coming into contact with built-up grease in the extraction system above the pizza oven.
A London Fire Brigade spokesperson said: "We'd like to remind restaurant and takeaway owners to ensure they don't have dirty ducting. If you don't clean the ducting in your extraction system regularly, you're at a greater risk of ducting fires.
"You should also make sure you keep the hob, cooker hood and extractor fan clean too – built up fat and grease can ignite and cause a fire."
The Brigade was called at 0326 and the fire was under control by 0505. Fire crews from Clapham, Battersea, Brixton and Lambeth fire stations attended the scene.