Six fire engines and around 40 firefighters were called to a fire at a superstore on Tollgate Road in Beckton.
A stack of pallets outside the building were destroyed by fire and a small part of the roof at first floor level was also damaged by the blaze. There were no reports of any injuries.
The fire was caused when a backup power generator overheated and set alight to nearby cardboard boxes and pallets.
Leading Firefighter Nick Matthews, who was at the scene, said: “Crews were initially called to reports of a bin alight and as we approached we saw an orange glow coming from behind a brick wall.
“Upon investigation, we found that it was actually a stack of pallets and cardboard boxes that were alight. They had been stored outside and placed too close to a generator, which overheated and ignited the pallets and boxes.
“Crews worked incredibly hard to bring the fire under control and prevent further damage.”
The Brigade was called at 2137 and the fire was under control by 0013 (Monday). Fire crews from East Ham, Plaistow, Barking, Dagenham and Ilford fire stations attended the scene.