Due to the severity of the crash, the northbound side of the road was closed but one lane has since reopened.
Delays on the A24 southbound are still being reported by the AA.
Delays of nine minutes have been reported on the A24 Findon Road southbound between A280 Long Furlong (Findon Roundabout) and A27 Warren Road (Offington Corner) but these are easing. The average speed is ten mph.
A Sussex Police spokesperson said: “Just after 6.10am on Wednesday (June 29) police responded to a report of a car leaving the road on the A24, Findon and colliding with a tree.
“The northbound side of the road has been closed while emergency services attend the scene.
“The driver of the vehicle was taken to hospital with life-threatening injuries.
“Anyone who witnessed the collision or has information which could help officers with their investigation is asked to report it to police by emailing [email protected] or call 101 quoting Operation Cranbourne.”
A spokesperson for West Sussex Fire & Rescue Service added: "At 6.17am we responded to a road traffic collision on the A24, Findon Road.
"Fire engines from Littlehampton and Worthing were sent to the incident, as well as the heavy rescue tender from Worthing.
"On arrival firefighters stabilised a vehicle and carried out an extrication of one casualty using hydraulic rescue equipment.
"The casualty was then left in the care of paramedics from South East Coast Ambulance Service and fire crews left the scene at around 7am."