Twenty firefighters attended the blaze in Muschamp Road after receiving numerous calls at about 5.54pm yesterday.
One person claimed the fire may have been started by youths on old furniture left outside the flat, although this has not been confirmed by the London Fire Brigade.
One resident said: "Some idiots in this world nothing better to do".
Four engines from Mitcham, Purley, Sutton and Wallington attended the second floor fire of the three story building and brought it under control by 6.17pm.
The front door sustained minor damage and parts of the flat suffered a small amount of fire damage.
No injuries were reported.
The cause of the fire is being investigated.
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