The air ambulance was called out to the incident at about 1am this morning.
Surrey Police remain at the scene.
It is not yet known when the major road will be reopened.
Traffic and travel service INRIX is reporting: “M25 anticlockwise closed, slow traffic due to accident investigation work from J12 M3 to J11 A320 St Peters Way (Chertsey).
“The road has been closed since 1am following a serious accident involving a van and a motorcycle. Expected to remain closed into morning rush hour.”
Highways England say traffic delays are currently over half an hour, with additional journey times expected on the diversion routes.
More to follow…