Firefighters tackled a fire at a house on Barchester Close in Uxbridge.
Part of a bedsit on the second floor of a three-storey house was damaged by fire. There were no reports of any injuries.
A London Fire Brigade spokesperson said: “A fire alarm fitted inside the property operated, raising the alarm and allowing time for three people to leave the building before the Brigade arrived.
“This incident shows how important it is to have smoke alarms fitted in your property. You should have working smoke alarms in every room where a fire can start, plus a heat alarm in the kitchen. They should be tested regularly."
The Brigade was called at 0230 and the fire was under control by 0305. Three fire engines and around 15 firefighters from Hillingdon, Hayes and Heathrow fire stations attended the scene.
The Brigade’s Fire Investigators believe the fire was caused by a faulty speaker device.