A 63-year-old woman has been left in a critical condition after she sustained head injuries in a crash on the M25.
The woman, from Watford, Herts, was a passenger in a Honda Jazz which was in collision with a Renault Clio.
Both car drivers were uninjured. Police closed junctions 29 to 30 for five hours after the Saturday night crash.
The Clio's driver, a 22-year-old man from Dartford, Kent, was arrested on suspicion of dangerous driving and released pending further inquiries.
A police spokeswoman said the 63-year-old woman remained in a critical condition at Queen's Hospital, Romford.