READ MOREEyewitnesses reported hearing explosions and fireballs "shooting into the air" from the factory in Waterloo Road, Staples Corner.
London Fire Brigade said 15 fire engines were sent to the scene, near the North Circular Road.
It said workers safely left the factory before crews arrived and the cause of the fire was being investigated.
Achal Dhillon, of Hendon, less than half a mile from the scene, said the fire was burning "quite violently".
The 33-year-old described seeing fireballs being shot up to 100 metres into the air from the burning building.
Fire crews were called at 21:20 GMT on January 8 and the blaze was under control by 00:22.
Smoke and flames could be seen for miles around and people were urged to avoid the area and those living nearby to keep doors and windows shut. ■