People are reported to be injured and motorists warned of delays following a "serious collision" on the M25 involving "up to seven vehicles".
Six ambulances and four fire engines are on the scene of the accident between junctions 10 and 11 in Surrey, in which two people are thought to have suffered "serious injuries".
Surrey Police said the clockwise carriageway is expected to remain closed "for some time".
In a statement, they said: "Surrey Police is currently at the scene of a serious collision involving a number of vehicles on the M25 motorway.
"Officers were called to the clockwise carriageway between junctions 10 and 11 near Addlestone following reports that up to seven vehicles had been involved in the collision.
"The clockwise carriageway is currently closed and is expected to remain so for some time while the emergency services, including the air ambulance, deal with the incident.
"Two people are reported to have suffered serious injuries while six others are reported to have suffered minor injuries. Motorists are advised to avoid the area and seek alternative routes wherever possible."
South East Coast Ambulance Service said they have "a number of resources at the scene".
A spokesman said: "We were called at just around 1.25pm reporting a multiple vehicle RTC on the M25 between junctions 10 and 11. We've got a number of resources at the scene including six ambulances, paramedics in cars plus the Hazard Area Response Team (HART)."
On its website the Highways Agency said: "On the M25 clockwise between junctions J9 and J10, there are currently delays of 30 mins caused by congestion due to a serious accident closing the road between junctions J10 and J11. Road expected to re-open from 3:45 pm."