We are appealing for information and witnesses following a single vehicle collision on the A3.
Police were called to the southbound carriageway just before 3pm yesterday (4 January) to reports of a road traffic collision.
A blue Nissan X-Trail left the carriageway just before the Queen Elizabeth country park.
The driver, a 55-year-old man from Chichester, was taken to Southampton General Hospital where he remains in a life-threatening condition.
The carriageway was closed for several hours as emergency services provided medical attention and investigated the scene.
Police Sergeant Spencer Wragg, of the Roads Policing unit, said: "We are interested in speaking to anyone who may have seen the blue Nissan travelling south on the A3. We believe it has travelled from the M25 on to the A3.
"We would particularly like to speak to anyone who has relevant dash cam footage of this vehicle or the incident itself.
"I would also like to thank motorists for their patience whilst we investigated this incident. We are aware of the impact this sort of closure has on motorists and we work hard with our partners to get roads reopened as quickly as possible, but we must also ensure our investigations at the scene are thorough for the sake of those injured and their families."
If you have information about this incident, please contact us on 101 quoting 44230004252.
Alternatively, you can submit information to us online here: https://www.hampshire.police.uk/tua/tell-us-about/cor/tell-us-about-existing-case-report/