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Eight fire engines and 58 firefighters and officers tackled a fire on Headstone Road in Harrow.
The fire affected a single storey air conditioning workshop and offices.
There were a number of gas cylinders involved in the blaze so a 75 metre hazard zone was put in place as a precaution as these cylinders can sometimes explode when exposed to heat.
Police assisted firefighters to evacuate properties in the immediate vicinity of the fire.
A London Fire Brigade spokesperson said: "The fire produced a lot of smoke in the local area so we advised people living and working nearby to keep their doors and windows closed. Firefighters will be at the scene throughout the night damping down pockets of fire."
The Brigade was called at 1504 and the fire was under control by 1842. Crews from Wembley, Hendon, Northolt and Stanmore fire stations were at the incident.
The cause of the fire is under investigation.