Cinders from a discarded cigarette butt set fire to a derelict hotel in Manor House on Sunday night.
The London Fire Brigade [LFB] were called at about 1am and the flames was under control two hours later.
It took 40 firefighters to tackle the blaze, which raged on the upper floors of the empty building in Seven Sisters Road.
A London Fire Brigade spokesperson said: “The blaze damaged half of the second floor and the roof.
“A small part of the first floor was also damaged by fire.
“The [LFB’s] fire investigation team believe the fire was accidental and due to the unsafe disposal of smoking materials.
“Fire crews from Holloway, Stoke Newington, Tottenham, Kentish Town fire stations attended the scene.”
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