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.
One traveler, Amy Capron, tweeted that it had taken her 17.5 hours since becoming stuck in traffic just 10 miles from the port before she was finally able to board her ferry.
"I normally love 'em, but today I am cursing the French!" she tweeted.
Kent police said Sunday that delays of up to 10 hours were still being experienced on the A20, and that disruptions were expected to continue for the next two days.
French border controls 'seriously understaffed'
The delays were due to French government requirements for heightened security checks on those entering the country following recent terror attacks, Highways England said in a statement. France has been under a state of emergency since the Paris terror attacks in November, when ISIS jihadists killed 130 people.
Highways England warned of major backlogs at the Port of Dover and the Eurotunnel, as Britons headed for European getaways at the start of the summer vacation season.
The Port of Dover said in a statement that the problem had been exacerbated by French border control posts being "seriously understaffed," with only three of a potential seven booths operating for tourists overnight.
"At one stage, only one French officer was available to check passengers on hundreds of coaches, resulting in each coach taking 40 minutes to process," said the statement.
It said the port raised concerns over the staffing levels to the British government earlier in the week, who in turn relayed them to French officials.
The UK Border Force is now being mobilized to help French officials clear the backlog, Britain's Home Office says.
On social media, some asked whether the lack of staffing at the border was a French payback for Britain's recent vote to leave the EU.
Authorities: Consider delaying travel
The Port of Dover advised passengers to consider delaying their travel or make sure they had adequate food and water supplies if they decided to travel anyway.
Ferry companies have said they will accommodate travelers who miss their slot due to the heavy traffic by placing them on the next available boat.
Emergency services and volunteers distributed water bottles to the motorists trapped in the summer heat, while some motorists used camping gear to cook meals as they sat in traffic.
At one point, a band of musicians delivered an impromptu roadside performance to entertain stranded travelers. Police have advised people to stay in their vehicles when stuck in traffic.
Charlie Elphicke, the MP for Dover and Deal, said that the "traffic chaos" was predictable and unacceptable, and that travelers were "owed an apology."