Nine people have been injured, two seriously, in a crash involving a minibus and a transit van on the M25.
The road was closed to allow an air ambulance to land on the carriageway after the crash at about 01:30 BST, close to junction 12 at Staines.
A man has been arrested on suspicion of dangerous driving, Surrey Police said.
Drivers faced continuing delays after the motorway being re-opened at 09:30 BST after the anti-clockwise stretch was partially closed during rush hour.
The force said the closure of the carriageway had been necessary to clear a fuel spillage and for the crash to be investigated.
Air passengers travelling from Gatwick or Heathrow airports were also warned to plan "significant extra time".
The congestion on the anti-clockwise carriageway is likely to take several hours to fully clear, so if you’re planning a journey before midday please allow extra time and plan an alternative route.— RPU - Surrey Police (@SurreyRoadCops) October 1, 2018