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A tunnel at the Dartford Crossing was closed after two lorries were involved in a crash.
Police and paramedics were called to the east tunnel at 11.20am following the crash on the M25
One of the drivers suffered a leg injury and were being treated by paramedics.
Kent Highways said motorists should expect long delays on the M25/A282 approach and in the surrounding areas.
At about midday, queues of four miles were being reported and an extra 30 minutes on journey times.
The tunnel had reopened by 2pm but hour-long delays were still being reported as the backlog cleared.