Motorists are being warned to expect the travel chaos to persist for over an hour following a nine-vehicle crash near Hatfield this morning.
The A1(M) southbound carriageway was closed between junctions one and three, causing long tailbacks to junction six for Welwyn.
There are also mile-long knock-on queues clogging up the M25 between Potters Bar and the A1(M) junction.
Highways England confirmed all lanes have now been cleared, so congestion on the M25 should ease, but warned knock-on traffic on the A1(M) between Hatfield and Welwyn is expected to persist until around 11.30am.
The chaos resulted in police setting up several diversion routes in WGC, which led to standstills in Bessemer Road, Woodhall and Stanborough.
Officers also closed the A1(M) entry slip road at Junction 4, causing congestion on the roundabout near Tesco, but that has now been lifted and traffic is expected to ease.
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