There is congestion to just after J19 (Catthorpe Interchange) Northbound, the A5 Northbound on the diversion routes, and on the A426 Northbound towards Dunton Bassett.
In a statement, Highways England have confirmed the accident happened on the Northbound side at around 15:30.
On their twitter page, Highways England said:
"Traffic caught up within the #M1 northbound closure between J20 (#Lutterworth) and J21 (#Leicester) is now being released in batches via the rear of the queue. This will take time, thank you for your continued patience."
The Southbound side was released at 16:25 after being held while the air ambulance landed.
What is the diversion?
There is a diversion in place via the A4303 Westbound, the A5 Northbound and the M69 Northbound which allows you to rejoin the M1 at Junction 21.
Exit the M1 northbound at J20 and take the first exit (west) onto the A4303.
Continue along the A4303 to the A4303/A426 Lutterworth Road/Rugby Road Roundabout and take the second exit from the roundabout.
Continue westbound along the A4303 passing through a further two roundabout at Coventry Road and Magna Park to A5/A4303 Cross in Hand Roundabout.
Take the fourth exit from the the roundabout and proceed north along the A5.
Continue northbound along the A5 Waitling Street to the A5/M69 J1 interchange.
At the interchange take the fifth exit onto the M69 northbound and continue along the M69 to M69 J2/M1 J20 and re-join the M1 northbound.