Firefighters were called to a blaze in a flat above a shop in Wandsworth High Street last night.
A passerby alerted the fire service to it at 9.15pm.
It was put out by 11.30pm after two fire engines were sent to the scene.
No one was inside at the time.
A spokesperson for the fire service said: "It is going down as suspicious and the cause is still being investigated.
"There were no working smoke alarms inside, but we have fitted them now."
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