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The Dartford Crossing was closed northbound following a serious crash between a lorry and car.
Emergency crews were called to the A282 on the Essex side of the crossing after the smash shortly before 5pm.
Both tunnels were shut by police, but the west tunnel reopened at about 5.30pm and the east tunnel reopened just after 7pm.
Essex firefighters and ambulance crews were also called to the scene.
Queues stretched back as far as junction 3 of the M25.