The road has been closed northbound between junctions 10 and 11 following a serious collision in the early hours of the morning (May 3).
Bedfordshire Police said it was called at about 3am to the two-vehicle crash. Four people have been taken to hospital with serious injuries.
The National Highways Agency says police have now completed their accident investigations but debris from the collision remains to be cleared from the road.
There are currently delays of 45 minutes above normal journey times as far back as junction 9.
And the closure is also impacting on roads through both Dunstable and Markyate as motorists seek alternative routes along the A5, and there is congestion on local roads through Luton along the diversion route from the motorway. The southbound carriageway has remained open during the incident.