People arriving at Dover for the Easter getaway have expressed shock and frustration at long delays - with some coach passengers facing waits of more than 14 hours.
Some coaches, including many carrying schoolchildren, had to wait overnight, and P&O Ferries said coaches were still facing three-hour delays.
People arriving in cars said they too were stuck in gridlock.
The port apologised for the "prolonged delays".
Long border processing times were to blame, it said, while some ferry companies said bad weather had disrupted travel.
Dafydd Francis, a PE teacher from Neath in South Wales, was part of a group of 33 children and adults who arrived at the port at 2300 BST Friday - and were still awaiting boarding 14 hours later.
He said he was "shell-shocked" by the delay. "We will arrive at the resort 14 hours late if we are lucky," he told the PA news agency.
"I have organised various trips since 1998 for school and family and friends, approximately 50 trips. We will fly next time."
P&O Ferries estimated delays of between one and two hours for cars trying to reach the port.
The port said car volumes had built through the morning and that there was a one-hour wait for them to get through border controls.
It advised passengers to check with their ferry operator for updates, allow plenty of time for their journey and "ensure they have some food, drink and entertainment with them".
It was "deeply frustrated", particularly on behalf of ferry operators' coach passengers "who have had to endure such a long wait", a spokesperson said.
Sara Miles from Tonbridge, who was travelling by car with her family to Normandy, said she was stuck for two hours in traffic outside the port, and that she was not sure when she would catch a ferry.
Ms Miles, who is going with her husband and two young daughters to visit her parents, told the BBC: "It's all a bit chaotic. People are turning off their cars and getting out, police are directing traffic.
"The girls are very excited about the holiday and it'd be too difficult to turn back now."
Another car passenger told the BBC that "the whole of Dover is practically gridlocked".
They said passengers were calm but that frustrations were building. "More and more people are getting out of their cars to try and find toilets - there are five portaloos from what I can see near the border control facilities."
P&O Ferries and DFDS Seaways have both reported disruption to Channel ferry crossings - with DFDS saying strong winds were adding to the problem.
In a statement to the BBC, the regional prefecture in Northern France said that there were "no difficulties that we know of", but that lots of coaches had arrived at the border in Dover at around the same time.
All border checkpoints were operational and border police had changed some car checkpoints into slots for coaches, it added.
A UK government spokesperson said it remained in close contact with ferry operators and authorities. Labour leader Sir Keir Starmer said the government needed to "get a grip" of the situation at Dover.
Simon Calder, travel correspondent at the Independent, said processing times since leaving the EU had increased sharply "and that would seem to explain the delays".
Speaking to BBC Breakfast, he said that having an EU border at Dover meant things were "gumming up", and that each individual passport must now be inspected and stamped.
This means that coach loads of passengers must disembark to have their passports checked, adding to delays.
Last year a critical incident in Dover was blamed on bad weather and a shortage of ferries at Easter.
The port said on Saturday that despite considerable pre-planning for this year, the additional coach bookings taken by ferry operators for Easter has impacted operations.
Food and drink had been provided to coach passengers caught up in the queues, the port said.
It was working to get "all passengers on their way as quickly as possible".
At 04:00 BST, the port reported delays of five to six hours for coaches - it later said that coach delays were still several hours long.
A coach of school children traveling to France had been given bags of crisps, some chocolate and water to see them through.
And a group of university students on a coach travelling to France arrived last night at 20:00 GMT after huge delays on the roads. They were kept in the cruise terminal at Dover and expected they would be waiting another 10 hours before they caught a ferry.
Student Oliver Quigley-Brown, who was travelling to France for a university ski trip, told the BBC he had been warned he faced a 14-hour delay.
He said he had seen "a lot of stressed teachers" at the port, estimating there could be "thousands" of people from school and university groups stuck there.
Ferry operators have been sending coach traffic to alternative waiting areas in order to clear the backlog of vehicles within the port.
DFDS UK tweeted early on Saturday that services to France were running with delays of up to 2.5 hours.
A spokesperson for DFDS earlier apologised for the wait times, which were blamed on bad weather delaying sailings as well as "high volumes of traffic... particularly coach groups".
In its own Twitter update, P&O Ferries stressed that it had "no control" over the border checks, but that coaches would be on "the next crossing to Calais after clearing check-in".
As well as the situation at Dover, there are fears of disrupted Easter getaways due to strikes affecting London's Heathrow Airport.
Hundreds of security officers in the Unite union have begun 10 days of industrial action over pay - though the airport said it was operating "as normal" on Friday.