Four fire engines and around 25 firefighters tackled a fire on Cleveland Street in Fitzrovia.
Half of a sauna in the basement of a mixed-use block was damaged by fire. Around 50 people left the building before firefighters arrived. There were no reports of any injuries.
The fire is believed to have been accidental and caused by items left on a sauna. A sprinkler system helped extinguish the fire.
A London Fire Brigade spokesperson said: “The numbers of buildings in London with sprinklers systems is still very low and we would like to see this increase. Automatic Fire Suppression Systems, like sprinklers, alongside other fire safety measures can greatly reduce the impact of fires on people, property, homes and communities.
“Sprinklers are not a luxury - they save lives and also reduce risk to the firefighters who have to tackle the fires.”
The Brigade was called at 1828 on Monday and the fire was under control by 1937. Firefighters from Euston, Soho and Paddington fire stations attended the scene.