Four fire engines and around 25 firefighters were called to a house fire on Shroton Street, Marylebone.
Most of a second-floor room in a property used as a HMO was damaged by fire. Ten people left the property before crews arrived. There were no reports of any injuries.
The fire is believed to have been accidental and caused by the unsafe disposal of smoking materials.
A spokesperson for London Fire Brigade said: "Smoking is still one of the leading causes of fire in London.
“While we’d rather you didn’t smoke at all, if you are a smoker, it’s vital you make sure that your cigarette is completely out when you’ve finished smoking it and disposed of responsibly. If you don’t, you risk causing a fire which could not only destroy your home but also cost you your life.
“A single careless act, such as a discarded cigarette, can have devastating consequences - threatening lives, destroying multiple homes and displacing families in a matter of minutes.
“Remember that non-smokers are reliant on others to put their cigarettes right out when they’ve finished them. Make sure you dispose of your smoking materials safely in an ashtray and never drop ash or throw cigarettes from balconies.
"Firefighters recommend that you have a look at our online Home Fire Safety Checker to make sure there are no hidden hazards in your home.
"The online tool will ask you a series of questions that will help us work out the level of risk in your home, or the home of someone you care for. It only takes a few minutes to get tailored advice to keep yourself and loved ones safe from fire.”
The Brigade’s Control Officers took the first of three calls reporting the incident at 1435 and sent crews from Paddington, Euston, Soho and surrounding fire stations to the scene. The fire was under control by 1511.