There were severe tailbacks on the M25 anticlockwise on the approach to the Dartford Crossing.
At the height of the queues, traffic had ground to a slow crawl from the tunnel back up to Junction 4 for Orpington.
There were also queues on the London-bound A2, where traffic joins the M25.
It was estimated there were waiting times of about an hour, which had been the case throughout the afternoon.
Initial reports of congestion, as the great summer holidays getaway got under way, came from Kent County Council's traffic cameras at 11.29am.
It is thought to have been caused by the weight of traffic.