Four fire engines and around 25 firefighters were called to a fire at an office block on Charterhouse Square in the Barbican.
A small part of an electrical substation in the basement of the seven-storey building was damaged by the fire. There were no reports of any injuries.
The Brigade was called at 0739 and the fire was under control by 1130. Fire crews from Shoreditch, Dowgate and Soho fire stations were at the scene.
The Brigade's Fire Investigators believe the fire was caused by a fault in a high voltage cable.