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A man had to be cut free from his car, a blue Renault Megane, after it was involved in a crash with a white Volvo articulated lorry.
The driver of the Megane, a man in his 20s, was seriously injured and taken to a London hospital by air ambulance, where he remains in a critical condition.
It happened around 5.20 this morning on the A20 London Road at its junction with Nepicar Lane, Wrotham Heath, near Sevenoaks.
A spokesman for Kent Fire and Rescue Service said: "Two fire engines attended and crews used hydraulic rescue equipment to remove the roof of the car freeing one casualty trapped inside,.
"He was handed into the care of South East Coast Ambulance Service paramedics and the air ambulance."
Kent Police said they had been called to the scene at just before 5.20pm.
A spokesman said: "A man sustained injuries and was taken to a London hospital."
The closure of the road, between the M20 and M26 roundabout, continued into the rush hour.
An alert from Kent Highways warned commuters to use an alternative route and expect delays in the Wrotham area.
The Kent County Council department, at about 7.40am, said that there were long delays on the A20 London Road and the A25 Maidstone Road.
At the same time police said that the A20 London Road remained closed between the motorway roundabouts while recovery work continued. Police say they hope tore-open it at 1.30pm.
Police are appealing for witnesses to the collision. They want anyone who saw the car or the lorry in the moments leading up to the collision, or who have dash cam footage from the area around the time, to contact them.
Witnesses can call the Serious Collision Investigation Unit on 01622 798 538, quoting reference SM/LB/016/20. Alternatively email sciu.td@kent.pnn.police.uk