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Eight miles of queues built up on the M25 after an accident on the QE2 Bridge near Dartford.
A car and a lorry are believed to have been involved in a collision on the clockwise carriageway at around 10.40am.
The outside two lanes, heading into Kent, were blocked.
It led to long delays for drivers entering the county, with queues stretching back beyond the Lakeside turn-off to Junction 29 for Romford.
All lanes have reopened by about midday but congestion took a couple of hours to subside.