The M1 is blocked between Edgware (j4) and Watford (j5) following a crash between four cars and a lorry.
Emergency services are at the scene treating multiple casualties after the crash in the northbound carriageway between a black Vauxhall Astra, grey Seat Ibiza, silver Mazda, white Toyota Prius and a white Mercedes lorry shortly before 1pm today.
Fire engines from Garston, Borehamwood, Stanmore, Mill Hill and Wembley have attended.
The Hertfordshire Air Ambulance and ambulance crews are also on site.
At this stage there are believed to be four casualties, the extent of their injuries is being assessed.
The grey Seat Ibiza was overturned in the collision.
Police are closing the slip road entry to the northbound carriageway.
The southbound carriageway has been reopened after it was closed temporarily to allow the air ambulance to land. It will close again when the ambulance takes off.
Highways England has warned motorists to expect delays.
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