Four fire engines and around 25 firefighters were called to a fire at a building on Hopton Street in Southwark.
A small part of an electrical intake room in the basement of a 21-storey building was destroyed by fire.
One man was treated on scene and taken to hospital by the London Ambulance Service.
The Brigade was called at 1214 and was under control by 1256. Fire crews from Dowgate, Lambeth, Dockhead and Soho fire stations were in attendance.
The cause of the fire is under investigation.