Story highlights Motorists complain of long waits in traffic to reach Dover ferries to France

Backlog blamed on heightened French border controls and understaffing at posts

London (CNN) Thousands of British travelers are experiencing a nightmare start to their European summer vacations as they find themselves stranded in huge lines of traffic trying to reach the port town of Dover.

Some motorists were forced to sleep in their cars as they became stuck in the backlog, caused by a glut of vacation season traffic encountering heightened security checks by French border officials at the port.

Dover, just 42 kilometers (26 miles) across the English Channel from the French port of Calais, is a key departure point for ferries to France, while the entrance to the Channel Tunnel is in nearby Folkestone.

French border officials, who are carrying out heightened checks in response to the terror threat in France, screen passengers in Dover before they depart from the UK.

P&O Ferries, which runs services from Dover, tweeted at one point early Sunday morning that delays on the A20 motorway were up to 12 hours, with a further two-hour wait once travelers reached the port.

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