The blaze at a commercial building on the North Circular in Neasden halted traffic on the ring road in both directions between Neasden Lane North and Staples Corner, where the key route meets the M1, in the early hours of Wednesday.
Transport for London (TfL) said the M1 had also been closed southbound at the interchange and urged motorists to use alternative routes.
The London Fire Brigade (LFB) said 10 fire engines and 70 firefighters were tackling the fire on the first floor of a nearby warehouse, while the Metropolitan police had sent officers to the scene.
Londons North Circular was closed in both directions after a fire broke out at a commercial building in Neasden, however it has since re-opened.
Scotland Yard tweeted: Were assisting @LondonFire at a warehouse fire on the A406 in #Neasden, Road closures likely to severely affect North Circ Rd, Staples Corner and other key routes into morning rush hour. Drivers should avoid the area.
LFB tweeted: Ten fire engines & around 70 firefighters are tackling a blaze in a commercial building on North Circular Road in #Neasden. Half of the first floor is alight.
AT LEAST 70 firefighters tackled an enormous blaze which has broke out in north London, next to the North Circular.
North Circular traffic latest: A406 closed as huge Neasden fire sparks rush hour chaos
Emergency services said 10 fire engines fought the blaze at a warehouse in Neasden, which started around 2.30am.
There are fears of travel chaos this morning as nearby residents are told to keep their doors and windows shut.
Neasden fire live: North Circular closed after firefighters tackle huge warehouse fire
The A406 North Circular is closed in both directions between Staples Corner and Neasden Lane because of the fire.
It is likely the major London ring road will be closed between the two junctions throughout the morning rush hour.
“Wed ask residents to keep their doors and windows shut as there is a lot of smoke in the local area”
In an earlier statement, London Fire Brigade said: “10 fire engines and around 70 firefighters are tackling a fire in a two-storey warehouse on North Circular Road in Neasden.
“Half of the first floor is alight. The A406 is closed eastbound in Neasden and the Staples Corner Flyover is also closed westbound.
“The Brigade was called at 2.38am. Fire crews from Willesden and surrounding fire stations are attending the scene.”
In a tweet, London Fire Brigade said: “As we continue to tackle the large warehouse blaze in Neasden wed ask residents to keep their doors and windows shut as there is a lot of smoke in the local area.”
It tweeted: “The warehouse fire in #Neasden is under control. Firefighters continue to deal with deep seated pockets of fire inside the building & road closures remain in place.
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