National Highways East reported the closure on Twitter at 11.30am.
It said: “Traffic has been stopped on the #M1 southbound between J8 #HemelHempstead #A414 and J6A #M25 due to a collision.
“Traffic officers are on scene and will try to move vehicles to a safe location. There are long delays with approximately 5 miles of congestion on the approach.”
More to follow.