A driver has been airlifted to hospital after the mini-bus he was in crashed into a bridge support on the motorway.
The collision took place at around 6.10am.
A Sussex Police spokesman said: “Emergency services were called to a road traffic collision on the M23 near junction 10 at Pound Hill, Crawley, which resulted in all northbound lanes being closed.
“Police are appealing for motorists travelling north to avoid the area, where congestion is already heavy, and to use an alternative route.
“A white Volkswagen Sharan people carrier collided with a bridge support, trapping the driver who was reported to have suffered serious injuries.
“Fire and rescue crews used cutting equipment to release the injured man, who was being treated by paramedics at the scene before being flown to St George's Hospital, London, by the air ambulance.
“The southbound M23 was closed for some 20 minutes to allow the air ambulance helicopter to land.”
Inspector James Biggs said: "We are advising northbound motorists to leave the M23 at junction 11 and travel via the A23 Brighton Road and the A2011 Crawley Avenue to rejoin the motorway at junction 10."
Anyone who saw the collision or who has other information which may assist an investigation into the cause is asked to phone Sussex Police on 101 or email email@example.com, quoting Operation Jevington.