Five fire engines and around 35 firefighters were called to a fire at a block of flats on Fulton Road in Wembley.
A small part of a first floor flat in a 21 storey building was destroyed by fire.
Fire investigators found the property didn't have working smoke alarms and the fire was most likely caused by the unsafe disposal of smoking materials.
A London Fire Brigade spokesperson said: "This incident shows just how vital it is to have working smoke alarms.
"It's also a timely reminder for smokers to ensure your cigarette is completely out when you’ve finished smoking it.
“If you don’t, you risk causing a fire which could not only destroy your home, but also cost you your life.”
The Brigade was called at 1805 and the fire was under control by 2034. Fire crews from Wembley, Park Royal and Stanmore fire stations were in attendance.