Two people have been taken to hospital "as a precautionary measure" after a multi-vehicle collision on the northbound carriageway this morning.
The people were taken to Watford General Hospital with neck injuries after five cars and a lorry collided.
The M1 has since re-opened after it was closed from junction three to allow the vehicles to be recovered.
Paramedics treated seven people in total for minor injuries at the scene.
Police said that debris covered about 100 metres of the carriageway.