A 15-year-old girl has been taken to hospital with serious head and internal injuries after a car crashed into a tree off the M4.
She was a back seat passenger in a white Toyota which left the westbound carriageway, hit a tree and overturned at about 15:00 BST on Sunday.
The male driver, 18, and a female passenger, 16, were not seriously hurt.
South Wales Police said the incident happened between junctions 34 (Miskin) and 35 (Pencoed).
Motorists faced delays after the crash and the westbound road was closed for three hours while investigations took place but has since reopened.
The 15-year-old was taken to the University Hospital of Wales, Cardiff.
Any witnesses have been asked to contact the police.