Motorists are facing long delays after the M1 was closed in both directions following a fire in a nearby scrap yard.
The motorway has been sealed off since 4.30am, between Junction 4, A41 (Edgware), and Junction 1, A406 (Staples Corner), over fears that gas cylinders at the yard may explode.
Police say the motorway could be closed for up to 24 hours to allow the cylinders to cool.
Traffic is queuing as far back as Watford on the M1 southbound, and the A1 and A41 diversion routes are also congested.
Drivers are being advised to follow black diamond diversion symbols in the area.
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