A man who was killed on the M25 was arrested on suspicion of drug driving before being hit by a car.
Officers from Essex Police stopped a vehicle and spoke to the driver in connection with motoring offences on the clockwise carriageway between junction 29 and 28 at around 12pm on December 12.
The man, in his 20s, was asked to take a roadside drugs test and was then arrested on suspicion of drug driving.
He was then involved in a crash with another vehicle and was pronounced dead shortly after.
His family have been informed of his death and a formal identification will take place in due course.
Essex Police has since referred themselves to the Independent Office for Police Conduct over their involvement.
Chief Superintendent Simon Anslow said: “Our thoughts are with the man’s family and friends at this difficult time.
“We have made a mandatory referral to the IOPC and it has decided to take on the investigation independently.
“We know this incident will be concerning to the public and we are being as transparent as possible at this early stage of the investigation.
“Our officers are witnesses to this tragic incident and are being supported.”
Any witnesses yet to speak with police are asked to call 101 reference CAD 2866/12Dec.
A spokesperson for the Metropolitan Police said: “Shortly before 12pm on Thursday, 12 December, officers were called to assist Essex Police at Junction 29 of the M25, following reports of a collision involving a car and pedestrian.
“Officers attended alongside other emergency services who treated the male pedestrian.
“Despite their best efforts, the man, aged in his 20s, was sadly pronounced dead at the scene.
“The man’s family have been informed of his death. Formal identification will take place in due course.
“Detective’s from the Met’s Roads and Transport Policing Command are investigating the collision.
“Any witnesses yet to speak with police are asked to call 101 reference CAD 2866/12Dec.”