Four fire engines and around 25 firefighters were called to reports of smoke issuing at an underground electrical sub station on Powis Street, Woolwich.
A low voltage board was alight and the power supply was isolated by UK Power Networks. There were no reports of any injuries.
The Brigade was called at 0934 and the fire was under control by 1256. Fire crews from Plumstead and East Greenwich fire stations attended the scene.
The fire is believed to have been caused by water coming into contact with an electrical supply.