Three cars and a van are said to be involved in the rush-hour smash, which has blocked the southbound carriageway close to junction 16.
A Highways England spokesman said at 8.30am: "All emergency services are now on scene and work is ongoing to treat injured persons and remove the affected vehicles form the scene.
"This is expected to take at least an hour."
Traffic is tailing back approximately seven miles to the M45, with delays of up to an HOUR.
Sensors also show traffic crawling on the A5 between Weedon Bec and Crick as drivers attempt to find an alternative route off the motorway