According to police, a white Mercedes C250 travelling westbound on the A259 is believed to have left the road, mounted the roundabout and hit a tree at about 1.34am.
Police say the driver, a 36-year-old man from Littlehampton, and his passenger, aged 32 from Worthing, both sustained life threatening injuries. They were taken by ambulance to the Royal Sussex County Hospital in Brighton.
The westbound lane of the road was closed for several hours while investigation work was carried out but has now reopened, according to police.
Police constable Steve Creal, said, “We would like to hear from anyone who saw the Mercedes being driven in the area before the collision or anyone who may captured what happened on dash cam.”
If you can help the investigation email [email protected] quoting Operation Reading.