A discarded cigarette sparked a balcony blaze in Heston on Monday morning.
Firefighters from Heston & Isleworth Fire Station were called to a first-floor flat at Fenton House, in Biscoe Close, at about 11am.
They had to break down the doors but were quickly able to extinguish the flames, which caused only minor damage.
Watch manager Mark Morrison said cigarettes were the main cause of domestic fires and urged smokers to take care when stubbing out.
"Even if you're smoking outside you still need to make sure you completely extinguish the cigarette," he added. "If we'd got there a few minutes later the whole room could have gone up in flames."
For a free smoke alarm and fire safety advice call London Fire Brigade on 0800 028 4428.