Nine people have been injured – two seriously – in a crash between a van and a minibus on the M25, police said.
Emergency services were called to a stretch of the London Orbital between junction 13 for Staines and junction 12 for the M3 at about 1.30am on Monday.
A man has been arrested on suspicion of dangerous driving, Surrey Police said.
The road was closed in both directions to allow an air ambulance to land on the carriageway, and the anti-clockwise lanes are still shut for investigations.
Diversions have been put in place.
Anyone with information or footage from the area at the time of the incident is asked to call police on 101.