The collision happened shortly before 3.30pm between junctions 14 and 15 and is believed to have involved two lorries.
Emergency services sped to the scene and three southbound lanes were closed to assist their progress. Fire engines were present to deal with a lorry that caught fire.
Delays of up to two hours were reported, and in Newport Pagnell, roads were gridlocked during rush hour as traffic avoided the M1.
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At 5.30pm, a National Highways spokesman said: “Traffic caught between J14 and the incident is now being released past the scene via lane 4. Delays of up to 60 minutes on the approach.”
At 7.35pm, the spokesman said: “The M1 northbound has now been reopened... Lanes 1,2 and 3 of (4) remain closed for vehicle recovery and resurfacing works following the fire. Delays of less than 10 minutes on the approach at this time.”