Traffic reports say one lane (of two) is closed southbound, with lengthy queues on the approach.
Congestion has also been reported at J11 (Pease Pottage).
The northbound carriageway closed earlier to allow an air ambulance to land.
Reports say this has now re-opened and the air ambulance has left the scene.
Read our updated story: Man taken to hospital after collision on A23 near Handcross