Four fire engines and 21 firefighters and officers were called to a fire at an industrial estate in Roebuck Road, Hainault.
An oxy-aceteylene welding pack (used for welding steel) was alight and crews cooled the cylinder to an ambient temperature.
Cylinders can explode when exposed to heat, so a 50m exclusion zone was put in place and around 50 people were evacuated from the surrounding area as a precaution.
The Brigade was called at 0820 and the fire was under control at 1125. Fire crews from Hainault, Romford and Dagenham fire stations attended the scene.
The cause of the fire is under investigation.