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Motorists are facing long delays due to roadworks at the Dartford Crossing today.
Two lanes are closed on the approach to the right-hand tunnel on the anti-clockwise carriageway.
The left hand tunnel, which is slightly smaller at only 4.8m high, remains open.
Traffic is building in the area with queues tailing back to the Darenth Interchange at junction 2.
Workers are carrying out ongoing maintenance on the tunnels.
National Highways says the work is expected to finish at 10am before another closure is put in place between 10pm and 5am tonight.
Drivers have been warned to expect delays and plan extra time for their journeys.