Six fire engines and around 40 firefighters were called to a fire in a boiler room at a gym unit in an underground shopping centre on Bank Street in Canary Wharf.
A boiler was damaged by fire. There were no reports of any injuries.
Smoke alarms fitted within the building sounded and alerted staff to the fire who then called 999.
The Brigade was called at 1730 and the fire was under control by 1940. Fire crews from Whitechapel, Plaistow, Dowgate, Poplar, Shadwell and Bethnal Green fire stations attended the scene.
The Brigade's Fire Investigators believe the fire was caused by a defect inside an electric water heater.