Eight fire engines and 58 fireman are tackling a fire at a builder’s merchant in Brent Cross.
A wood working workshop is currently alight but the cause of the blaze is currently unknown.
The London Fire Brigade were called to the scene at 3.45pm and received more than 15 calls to report the incident.
The M1 was closed both ways between the A41 Watford Way and the North Circular Road to allow emergency services to deal with the building fire as smoke is blowing across the road.
Witness Gavin Ellis was travelling on a train passing the scene of the fire. He wrote on Twitter: "Train driver just warned us to shut windows as we passed a large fire. Didn't disappoint. #brentcross #fire #smokey"
Junction 4 heading southbound is also affected.
The smoke can be seen from near the motorway near Staples Corner.
Thick black plumes of smoke billowed into the sky as firefighters tackled a huge blaze in Brent Cross.