Ten fire engines and around 70 firefighters were called to a fire at a residential block of flats on Royal Drive in New Southgate.
Part of two flats with mezzanine levels on the fourth floor of the building were damaged by fire. The fire also spread and damaged the roof structure directly above the two flats.
Around 25 people left the building before the Brigade arrived. Firefighters and London Ambulance Service crews assisted one woman from the third floor via an internal staircase. There were no reports of any injuries.
The fire was discovered by a resident who was in a bedroom on the upper floor and smelt burning. They looked out of the window and saw smoke and attempted to extinguish the fire before leaving the property.
The Brigade's Fire Investigators believe the fire was caused by the unsafe disposal of smoking materials.
A London Fire Brigade spokesperson said: "We’d rather you didn’t smoke at all, but if you are a smoker, 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 not only destroy your home, but also cost you your life.
"This is also a reminder to never try to tackle a fire yourself. Leave it to the professionals and call 999 as soon as you become aware of a fire in your home."
The Brigade was called at 1148 and the fire was under control at 1545. Fire crews from Southgate, Finchley, Hendon, West Hampstead and surrounding fire stations attended the scene.