Four vehicles – a Ford Transit, a delivery van and two cars – were involved in the incident, which occurred around 12.24pm on Thursday 17 June.
Two people have been taken to hospital; a man with life-changing injuries and a woman with life-threatening injuries. No other injuries were reported.
The A27 and A26 have reopened on Friday 18th June following investigations and recovery work completed. This includes debris and damage to the road surface.
Police are urging anyone who saw what happened – or anyone with relevant dash cam footage – to email firstname.lastname@example.org quoting Operation Craven.
Police would also like to thank the public for their patience and understanding when the road was closed.