A woman has died after a multi-vehicle crash which involved a van that had previously been chased by the police.
Officers from Hertfordshire Constabulary were involved in the pursuit in the early hours of Sunday, before it was called off.
The van was then involved in the deadly collision with three other vehicles, between the junctions for Watford and St Albans on the M25.
The Independent Office for Police Conduct (IOPC) has since launched an investigation.
A Hertfordshire Constabulary spokeswoman said: "The M25 remains closed between junctions 22 (London Colney) and 21A (North Orbital Road/St Albans) following a collision at around 4am this morning.
"Officers were in pursuit of a van before the van was involved in a collision with three other vehicles.
"No police vehicles were involved in the collision and the pursuit had been stood down prior to the collision.
"A woman has sadly passed away and her family are being supported by specially trained officers.
"As a matter of course, the incident has been referred to the Independent Office for Police Conduct."
An IOPC spokeswoman said: "We are investigating the police actions prior to a fatal road traffic collision on the M25 earlier today.
"We understand around 4am this morning Hertfordshire Constabulary were in pursuit of a van before the van was involved in a collision.
"The incident, involving three other vehicles, was between Watford and St Albans. No police vehicles were involved in the collision and the pursuit had been stood down prior to the collision.
"We sent our investigators to the scene and to the post-incident procedure to begin the process of gathering information.
"Our investigation is in the very early stages."