The M25 is closed anticlockwise at junction 18 due to a serious accident involving two lorries, a motorcycle, car and a van this morning.
The incident near Maple Cross, Hertfordshire, is causing congestion near to Heathrow Airport.
Traffic is queuing back to junction 15 and diversion routes have been put in place by Highways England, reports Surrey Live.
The Transport for Bucks Twitter account warned shortly after 9am of "almost total gridlock" across the A roads in the south of the county, affecting Amersham, Chesham, Denham and Iver.
Traffic was previously held for 35 minutes on the M25 clockwise between junction 16 and 17 for an air ambulance to land.
All lanes reopened shortly before 9am after traffic was held for around 35 minutes for the air ambulance to land.
It has now left the scene, but the M25 anticlockwise remains closed.
Travel news service INRIX says: "Stationary traffic due to all traffic being temporarily held and serious accident on the other side on M25 clockwise from J16 M40 to J17 A412 Denham Way (Maple Cross)."
A spokesman for Highways England said: "The M25 in Hertfordshire is closed anticlockwise between junction 18 (near Chorley Wood) and junction 17 (Mill End) due to a serious road traffic collision.
Hertfordshire Police, emergency services and Highways England traffic officers are currently in attendance.”
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