Four fire engines and 21 firefighters and officers were called to a fire at a telephone exchange building in Blackfriars Road, Southwark.
The fire was in a plant room on the second floor but was out on the arrival of crews. There were no reports of any injuries.
The Brigade was called at 2130 and the incident was over at 2331. Fire crews from Dowgate, Dockhead, Old Kent Road and Shoreditch fire stations attended.
The cause of the fire is believed to have been an electrical fault within a computer bank.