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Part of the Dartford Crossing was closed due to a three-vehicle crash.
Motorists on the M25 were warned about long delays while emergency services dealt with the incident, which saw five people taken to hospital.
The northbound westbore tunnel was shut at around 3pm due to the collision, and reopened around two hours later.
Queues on the M25 are stretching back past junction 3 for the Swanley Interchange.
National Highways said: “Emergency services are in attendance. All vehicles are being diverted through the eastbore tunnel.
“Long delays are expected on approach, allow extra journey time.”
Police, the fire service and ambulance service have been approached for more information.