Four fire engines and around 25 firefighters were called to a fire at a house converted into flats on Arundel Square in Islington.
A flat on the third and fourth floors plus an internal staircase leading from the third level to the fourth, in the five storey building, were completely destroyed by fire.
Three people left the building before the Brigade arrived.
The Brigade was called at 0836 and the incident was under control by 1024. Fire crews from Islington, Holloway, Euston, Kentish Town fire stations were in attendance.
The cause of the fire is under investigation.