Four fire engines and around 25 firefighters tackled a flat fire on Belsize Park Gardens in Belsize Park.
Part of a flat on the second floor of a converted semi-detached house was damaged by fire. There were no reports of any injuries.
The Brigade was called at 0807 and the fire was under control by 0918. Fire crews from Kentish Town and Euston fire stations attended the scene.
The cause of the fire is believed to have been accidental and involved a microwave.