Six fire engines and 35 firefighters and officers were called to a fire at a warehouse with offices above on Haldane Place in Wandsworth.
A security guard completing external checks saw smoke coming from the second floor windows and immediately called 999. Eight sprinkler heads went off in the room of origin and helped contain the fire damage to that room.
The fire is believed to have been caused by sunlight reflecting off mirrors onto an unknown combustible item, possibly window blinds.
A London Fire Brigade spokesperson said:
“The sun is especially strong at the moment but fires caused by refracted sunlight can happen all year round and are actually quite common.
"Our advice is to make sure that you keep mirrors, crystals, glass ornaments and other reflective items out of direct sunlight at all times.”
A small section of the second floor was damaged by fire. One woman left the premises before the arrival of the Brigade.
The Brigade was called at 0914 and the fire was under control at 1109. Fire crews from Wandsworth, Tooting, Battersea, Clapham, Chelsea, and Mitcham fire stations attended the scene.