Six fire engines and around 40 firefighters were called to a fire at a takeaway restaurant with flats above on Station Road in Edgware.
Part of a restaurant on the ground floor of the building and part of a four roomed flat on the first floor were damaged by fire. Most of the roof was also damaged by the blaze. Six residents and two members of staff left the building before the Brigade arrived. There were no reports of any injuries.
The fire was discovered by a member of the public who looked out of their window and noticed smoke coming from the roof of the building.
The Brigade's Fire Investigators believe the fire was accidental and caused by oil igniting in a deep fat fryer, which then spread via heavy grease within the extraction system.
A London Fire Brigade spokesperson said: “We'd like to remind restaurant and takeaway owners to make sure that they don't have dirty ducting.
"If you don't clean your extraction system regularly, you're at a greater risk of ducting fires.”
More information on using deep fat fryers safely
The Brigade was called at 1532 and the fire was under control by 1658. Fire crews from Stanmore, Mill Hill, Hendon and Harrow fire stations attended the scene.