Four fire engines and around 25 firefighters were called to first floor flat fire on Cromwell road in Hounslow.
Most of the of the ground floor of the property was damaged by the blaze, as well as some of the first floor. A man and woman left the property before the Brigade arrived.
The Brigade was called at 1055 and the fire was under control by 1229. Fire crews from Heston, Kingston and Feltham attended the scene.
The cause of the fire is under investigation.