Five fire engines and 31 firefighters and officers were called to a fire at an office block in Undershaft in the City.
Part of a computer switch room in the basement of the building was damaged by the fire. Around 20 people left the building before the Brigade arrived. There were no reports of any injuries.
The Brigade was called at 1044 and the fire was under control at 1400. Fire crews from Lambeth, Dowgate, Dockhead and Old Kent Road fire stations attended the scene.
The cause of the fire is believed to be a malfunction in a UPS (uninterruptible power supply) inverter.