Stationary vehicles tailed back up to 12 miles (19 kilometres) inland from Dover, on England’s southeastern tip.
The peak summer holiday getaway season and what Dover port officials said was a lack of French border control staff combined with the increased security to create the mammoth queues.
Dover is Britain’s main ferry port to continental Europe, with Calais in northeastern France 21 miles (33 kilometres) away across the Channel.
A multiple sclerosis sufferer travelling to Germany for stem cell treatment was among those forced to spend the night in their vehicles.
What should have been a straightforward journey to Dover turned into a 20-hour ordeal for 50-year-old Tanya Cudworth, who was travelling to a Frankfurt clinic.
She told the Press Association news agency that her experience was “absolutely horrendous”. “Nineteen hours in the car has obviously aggravated my symptoms,” she said.
“During the day it was so hot and there was nowhere near enough water and at night… you couldn’t sleep because you had to keep moving forward.
“We didn’t get any water until 3 am and I saw women with babies, young families and people with pets with no water. It’s shocking that more wasn’t done to get it to people.”
‘Very miserable day’
People were advised to bring food and drink supplies, while Sikh humanitarian organisation Khalsa Aid delivered bottles of water and snacks.
“We met a lot of young families with children, mostly people going on holidays,” said founder Ravi Singh, 46.
“People didn’t know what was going on.
“People were very, very frustrated and pulling their hair out.
“It was a very miserable day for many people.”
Dover port authorities said French border control booths had been “seriously understaffed overnight”.
British border officials were drafted in to help their French colleagues.
“We recognise the security pressures that French law enforcement organisations are under at Dover,” said a British government spokeswoman.
“There has been extraordinary disruption in the Dover area today but safety is paramount.”
Highways England, which runs the road network, said the delays were due to “heightened security checks to keep the travelling public safe following the recent attacks in France”.
By 5 pm (1600 GMT) Sunday, the local Kent Police force said traffic had returned to normal levels, with delays down to around 30 minutes.
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.
“Given the European context and the prolonged state of emergency, officers are obliged to check every vehicle rigorously.”
The Independent online newspaper said the queues “may be the first sign of what it means to live outside the European Union”.
“The snakes of traffic outside Dover are a reminder that we cannot expect life to carry on as normal after Brexit,” its editorial said.