Four fire engines and around 25 firefighters were called to reports of smoke issuing at a sewage works in Jenkins Lane, Barking.
Firefighters wearing breathing apparatus carried out a thorough search of a detached two-storey building on the site. A fire in some underground cabling had already self extinguished but caused heavy smoke-logging throughout the building.
The Brigade was called at 0848 and the incident was over for crews by 1215. Fire crews from East Ham, Barking, Plaistow and Ilford stations were at the scene.
The cause of the fire is believed to have been an electrical fault in an underground high voltage cable.