Update: Police re-open all M1 lanes
All lanes of the M1 northbound have now re-opened following a severe accident - but there are still delays of up to an hour.
Police shut off the entry slip road of junction 13 at Milton Keynes this morning, but say traffic conditions should be back to normal by 1.30pm.
A diversion was in operation via the A421 into Milton Keynes, the A4146 towards Northampton, and A509 back to the M1 at J14.
There are currently delays of up to an hour between junction 12 and 13 - and up to 30 minutes on the southbound carriageway as drivers slow down to look at the accident.