Three people have died and two drivers have been arrested following a serious collision on the M25 involving a lorry, a minibus and a car, police have said.
A fourth person is in hospital with life-threatening injuries after the crash, which took place near Waltham Abbey at around 6.15pm on Monday.
Essex Police said in a statement the collision occurred between junctions 27 and 26 of the anti-clockwise carriageway, with two drivers arrested on suspicion of causing death and serious injury by dangerous driving.
The M25 was closed for some 11 hours between those two junctions, before opening soon after 5am on Tuesday.
Officers said they were keen to trace the movements of all three vehicles - a Ford Mondeo, a minibus and a lorry - leading up to the collision.
Earlier, the ambulance service said "a number of people" were being treated following the crash between junctions 27 and 26 of the anti-clockwise carriageway.
Parv Domah was among those caught up in the traffic tailbacks near junction 26, telling PA news agency: "I can't tell you exactly what happened, I can only say I've been here going on three hours.
"There were approximately 30 emergency vehicles that drove past in that time, as well as two air ambulances.
"Traffic was stopped on both sides of the carriageway at one point and now on the anti-clockwise is still shut."
East of England Ambulance Service NHS Trust said in a statement: "We are at the scene of a serious collision on the M25 near Waltham Abbey.
"We have sent our Hazardous Area Response Team, two air ambulances, nine ambulances, four ambulance officers and a LAS (London Ambulance Service) vehicle.
"Both carriageways have closed and we are treating a number of people involved."
Fire crews attended the scene from Brentwood, Loughton, Waltham Abbey and Epping.
Incident Commander Richie Green, of Essex County Fire and Rescue Service, said: "Our crews worked extremely hard in a difficult and dynamic situation.
"We thank the public for their patience while we worked with other emergency services to make the scene safe."
Police are urging witnesses with dashcam footage to submit a report here, or call 101 citing incident 972 of Monday, August 23.
Information can also be given to Crimestoppers anonymously on 0800 555 111 or here.