Six fire engines and around 40 firefighters tackled a fire on The Downs in Wimbledon.
Most of a fourth-floor flat in a four-storey residential block was damaged by fire. Twenty-seven people evacuated the building. There were no reports of any injuries.
The fire is being treated as accidental and is believed to have been caused by sunlight refracting off a mirror onto a curtain.
A London Fire Brigade spokesperson said: "These sorts of fires are not as rare as you would think. They can happen all year around, but particularly in the summer when the sun is stronger, it’s really important that reflective items such as mirrors, crystals and glass ornaments are kept out of direct sunlight."
The Brigade was called at 1123 and the fire was brought under control by 1217. Crews from Wimbledon, New Malden, Wandsworth, Mitcham and surrounding fire stations have been in attendance.
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