Forty people managed to escape from their hostel when a fire started in the nine-storey building.
A woman needed to be treated by London Ambulance Service for slight smoke inhalation but did not need to be taken to hospital.
There are no reports of anyone else being affected by the smoke or flames from the fire at the property in Dingwall Road, Croydon.
The fire started in a studio flat on the third floor with about half of the room damaged by the flames and smoke.
A London Fire Brigade spokesman said it is believed to have been started by the careless disposal of a cigarette.
They added: “It’s absolutely vital you ensure your cigarette is completely out when you’ve finished smoking it.
“If you don’t, you risk causing a fire which could destroy not only your home, but also your life.”
Four fire engines and 21 firefighters and officers from Norbury, Addington and Beckenham fire stations attended the incident.
They were called at 6.19pm on Friday, July 3, and had the fire under control an hour later.
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