Four fire engines and around 25 firefighters were called to a fire at an office block on Broadgate in The City.
A small part of a ground floor server room in an eight-storey block was damaged by the fire. Six people left the building before the Brigade arrived. There were no reports of any injuries.
The Brigade was called at 0857 and the fire was under control by 1048. Fire crews from Shoreditch, Islington, Holloway and Stoke Newington fire stations were at the scene.
The fire is believed to have been caused by an electrical event in a power supply unit.