Eight fire engines and around 60 firefighters tackled a fire on Weir Road in Balham.
Nearly half of the ground floor of a two-storey industrial launderette was damaged by fire. There were no reports of any injuries.
The fire is believed to have been accidental and caused by the self-heating of laundry.
A London Fire Brigade spokesperson said: "These types of fires often involve textiles that become contaminated with oil, most commonly linseed, massage and cooking oil on tea towels, tablecloths and chef whites. Make sure you check the manufacturers cleaning recommendations of your textiles before washing.
"Sometimes when materials are cleaned and put in tumble dryers, the heat from the tumble drying cannot escape. This can result in a high enough temperature allowing it to build up to a point where it smoulders and eventually ignites.
"If you are washing and then drying on a hot cycle, always use the cooling cycle on the tumble dryer so it cools down and allow the heat to dissipate properly, before stacking laundered items together.”
The Brigade was called at 0108 and the fire was brought under control by 0206. Crews from Clapham, Battersea, Brixton, Tooting and West Norwood fire stations attended the scene.