Four fire engines and around 25 firefighters were called to a basement flat fire on Westbourne Street near Hyde Park.
Part of the basement flat in a four storey mid-terraced house, which has been converted into flats, was destroyed by fire.
Two people left the building before the Brigade arrived. They were suffering from smoke inhalation and were treated by London Ambulance Service.
There were no working smoke alarms at the property.
A London Fire Brigade spokesperson said: "This incident shows just how important it is to have working smoke alarms fitted on every level of your property.
“Smoke alarms give the earliest possible warning when a fire starts and we would urge everyone to make sure they have one fitted in every room where a fire can start except kitchens or bathrooms where heat alarms are more appropriate.”
The Brigade's 999 Control officers were called at 2014 and mobilised crews from Paddington, Kensington and Euston fire stations. The incident was over by 2134.
The cause of the fire is believed to be accidental and undetermined.