Northbound traffic is being diverted via the Hollow Diamond symbol.
Northbound traffic is being diverted left onto the A509 Portway, continuing on the A509 to the junction for the A5/A508 Old Stratford Roundabout.
Drivers are then being asked to take the third exit onto the A5 northbound, continuing on the A5 through Potterspury and Towcester to the A43/A5 roundabout before continuing to the junction with the M1 junction 14 and rejoining the M1 southbound.
Southbound traffic is being diverted via Hollow Circle symbol.
Motorists are advised by Highways England to take the A508 southbound through Roade and Grafton Regis to the A508/A5 Old Stratford Roundabout before taking the A5, continuing to the A5/A509 Portway Interchange.
Then vehicles should continue to the junction with the M1 junction 14, rejoining the M1 southbound.
Traffic traversing between London and the Midlands may wish to consider using the M40 as an alternative strategic route, Highways England says.
Traffic trapped between junctions 15 and 14 on the southbound side is now, slowly, being led back up the northbound side in single file.
Those motorists have now been stranded between the Northamptonshire border and Newport Pagnell for close to eight hours.