Four fire engines and around 25 firefighters were called to a fire at a block of flats on Adelaide Road in Belsize Park.
Most of a second floor flat was completely destroyed by fire. There were no reports of any injuries.
The Brigade was called at 1522 and the fire was under control by 1604. Fire crews from Kentish Town and West Hampstead fire stations were at scene.
The cause of the fire is under investigation by the Brigade and the Metropolitan Police Service.