Four fire engines and around 25 firefighters were called to a flat fire on North Hill in Highgate.
Most of an electrical intake room in the basement of a four-storey block of flats was damaged by fire. There was heavy smoke logging and firefighters wearing breathing apparatus evacuated around 20 people from the ground and first floors whilst crews ventilated the building. There were no reports of any injuries.
The Brigade was called at 0433 and the fire was under control by 0601. Fire crews from Hornsey and Kentish Town fire stations attended the scene.
The Brigade's Fire Investigators believe the fire was accidental and caused by the electrical failure of the main electric service supply.