A coach with 35 passengers on board overturned as it left the M40 motorway.
Seventeen people were taken to hospital and four had surgery for injuries including broken bones. The vehicle turned on its side at 11pm on Saturday at the exit of junction seven, near Thame in Oxfordshire.
It was about half full, with 33 passengers and the driver.
All the casualties were taken to John Radcliffe Hospital in Oxford.
Six ambulances from South Central Ambulance Service attended the scene.
Police said five people remained in hospital today with serious injuries, though none were believed to be life threatening.
A police spokeswoman said: "Although the cause of the incident remains under investigation, officers do not believe that any other vehicle was involved."