Stationary vehicles could be seen snaking through the countryside, inland from Dover at England’s southeastern tip.
The town is Britain’s main ferry port to Calais in northeastern France, 21 miles (33 kilometres) across the Channel.
Under a reciprocal agreement, Britain and France have border controls on the opposite side of the Channel.
“We recognise the security pressures that French law enforcement organisations are under at Dover and we have agreed the UK Border Force will assist the PAF (French frontier police) with border checks to remove the backlog,” said a spokeswoman for Britain’s Home Office interior ministry.
“There has been extraordinary disruption in the Dover area today but safety is paramount.”
Some people slept overnight in their cars.
Highways England, which runs the road network, told drivers: “French border police at the Port of Dover and Eurotunnel continue to follow French government requirements to deliver heightened security checks to keep the travelling public safe following the recent attacks in France.”
The local Kent Police force said there was a 12-mile tailback, which it put down to a “vast volume of holiday traffic”.
People were advised to bring food and drink supplies, while a Sikh humanitarian organisation delivered water and snacks.
Xavier Czerwinski, a senior official from the Pas-de-Calais area, said: “The situation is exceptional because it’s the weekend when Britons make the great getaway to the continent.
“We have deployed the maximum resources.
“Given the European context and the prolonged state of emergency, officers are obliged to check every vehicle rigorously.”