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A woman suffered life-threatening injuries following a crash between a car and a lorry on the Dartford Crossing.
The 23-year-old from Essex was taken to a London hospital with life threatening injuries following the collision, which happened on the Kent side of the crossing at around 4.30am.
It led to the closure of the bridge, which did not reopen until around 1.40pm.
The closure caused traffic chaos with motorists heading to Kent from Essex being diverted through the east tunnel, meaning only one tunnel was in use for drivers leaving Kent.
The tunnel reopened at 2pm.
It has caused massive delays on the M20 and M25, with queues stretching back to junction 4 of the M25 for Orpington.
In the other direction, delays in Essex were reaching J28 for the Brook Street roundabout.
Police said the crash involved a silver Audi A1 and a commercial lorry.
A spokesman said: "Officers would like to hear from anyone who saw the lorry, which is a white in colour with red and blue writing on the side, and which was towing a drawbar trailer, in the moments before the collision."
A 25-year-old man from Croydon who was arrested on suspicion of causing serious injury by dangerous driving has been released under investigation pending further enquiries.
KCC Highways said: "The tunnels are now returning to normal operation.
"Long delays remain on both approaches.
"There is queuing traffic on the M25 Anticlockwise from M25 J3. There is queuing traffic on the M25 Clockwise from the M25 J29."
Anyone with information can call Kent Police’ Serious Collision Investigating Unit on 01622 798538 or email sciu.td@kent.pnn.police.uk quoting reference number DJ/250/17.
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