A collision involving eight cars and a horsebox on the M25 brought motorway traffic to a standstill.
Two people were treated at the scene of the crash, near Heathrow Airport on the clockwise carriageway near Junction 15.
London Ambulance Service (LAS) said paramedics were called at about 11:00 BST to the scene, where delayed drivers had got out of their vehicles.
Highways England said the motorway was later reopened and traffic was on the move again.
One of the two people treated at the scene was taken to hospital, LAS said. Their condition is not known.
The vehicles involved, including an empty horsebox, were moved to the hard shoulder within about 30 minutes, a spokesman added.
Motorists said they were delayed by at least 45 minutes.