The A1(M) was initially closed in both directions tonight to allow an air ambulance to land after a serious crash near Stevenage.
The southbound section of the motorway still remains closed following the crash, which occurred between Junction 8 and 7 for Stevenage.
Police and ambulance crews are still on the scene, but traffic has now been released northbound.
Traffic has been backing up past Junction 9 at Letchworth Gate southbound, with the northbound carriageway still heavily congested despite reopening.
There are currently no details about any injuries, but the incident has been described as serious.
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