Traffic is crawling for five miles on the M1 southbound from Northampton after a crash near Milton Keynes at Monday lunchtime (July 5).
A lorry and car are reported to be involved in the smash near to Newport Pagnell Services.
Highways England is warning those heading south from junction 15 face delays of up to 30 minutes.
There are queues at the junction 15 southbound exit slip as drivers attempt to avoid the congestion and on the A45 as others struggle to join the motorway.
A spokesman for the agency said: "Our traffic officers and Thames Valley Police are currently at the scene. Recovery are on their way to the area to assist but we anticipate delays could last until mid-afternoon."