The incident took place at around 4.20pm on Tuesday 25 June 2019 on the coastbound carriageway and involved a blue Audi A4 and a black Infiniti M35h.
It is understood other as yet unidentified vehicles may also have been involved in the collision.
Officers, the South East Coast Ambulance Service and the air ambulance attended the scene and a man in his 60s was taken to a London hospital with injuries. He remains in hospital where his condition is described as serious.
Officers are working to establish the circumstances of the incident and are urging anyone who saw the cars in the moments leading up to the collision, or who has dash cam footage from the area around the time, to contact them.
They would also like to hear from any drivers who were involved in the collision and have not yet spoken to a police officer.
The coastbound carriageway was closed while emergency services dealt with the incident and the circumstances of the collision were examined by our Serious Collision Investigation Unit. It reopened at around 10.40pm on 25 June. The Londonbound carriageway was also closed to allow the air ambulance to land at the scene.
Witnesses can call the Serious Collision Investigation Unit on 01622 798 538 quoting 25-1172. Alternatively email email@example.com.