The Port of Dover has declared a "critical incident" due to huge traffic queues affecting holidaymakers.
The seaport's boss, Doug Bannister, said they have been "badly let down" by "woefully inadequate" and "insufficiently resourced" French border controls in Dover.
Travellers have been warned not to head to the Kent port on what is expected to be one of the busiest days on the roads in the UK as summer holidays start for millions of kids.
Delays at Dover are also causing tourist and freight traffic to be stuck on gridlocked roads in the area.
In a statement released today, the port said: "We are deeply frustrated that the resource at the French border overnight and early this morning has been woefully inadequate to meet our predicted demand and even more deeply regret the consequences that will now be felt by so many."
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Georges-Francois Leclerc, the prefect for the Haut-de-France region, hit back at Mr Bannister, saying: "The increase in traffic for this weekend was fully anticipated and a suitable deployment was prepared. Based on a close analysis of predicted traffic, the plan was to have all the police booths manned (at Dover) by 8:30am. However an unforeseeable technical incident in the tunnel meant that police had to push back their full deployment by an hour. It was not until 9.45am that the booths were fully operational."
Holidaymakers booked on sailings reported being stuck in five-hour queues to complete border checks before they can check in for their ferry and the Port of Dover has warned tomorrow will also be a busy day.
The port added: "We have worked particularly hard, and extremely positively up to now, with our Police Aux Frontieres (PAF) colleagues over recent months to plan for the traffic volumes that were fully expected.
"We have shared traffic volumes in granular detail with the French authorities in order that these volumes can be matched by adequate border resource.
"The Dover route remains the most popular sea route to France and France remains one of the key holiday destinations for British families. We know that resource is finite, but the popularity of Dover is not a surprise.
"Regrettably, the PAF resource has been insufficient and has fallen far short of what is required to ensure a smooth first weekend of the peak summer getaway period."
The port plans to "liaise constructively" with French partners, but warned the system is "reliant on every team member playing their part."
The Foreign Secretary has labelled the delays and queues at Dover "unacceptable", calling it an "entirely avoidable" situation.
Liz Truss said in a statement: "This awful situation should have been entirely avoidable and is unacceptable.
"We need action from France to build up capacity at the border to limit any further disruption for British tourists and to ensure this appalling situation is avoided in future. We will be working with the French authorities to find a solution."
Leader of Dover District Council, Cllr Trevor Bartlett, told of the dismay, desperation, and anger in Dover as the town remains grid locked on day one of the big summer getaway.
He said: "Hours of planning by the Kent Resilience Forum for the busiest weekend of the year for cross-Channel travel have gone out of the window within hours, with the Port of Dover calling a critical incident.
"Alongside the delays for tens of thousands of tourists, the local community is the biggest loser. Residents can't get to work, children are stranded on school buses stuck in the mayhem, and businesses are suffering. Stagecoach has suspended bus services in Dover altogether.
"I wrote to the Kent Resilience Forum three weeks ago to express my concerns and to call for a more proactive approach to keep Dover clear. Those calls were not heeded.
"Serious questions need to be asked about how the emergency services would be able to respond to a major incident in Dover when it is completely grid locked, and how this situation has been allowed to develop yet again?"
Meanwhile, fuel price protests are set to cause disruption during what is expected to be the busiest summer getaway on the roads in at least eight years.
Avon and Somerset Police warned motorists that a "slow-moving protest convoy" of around 10 vehicles has joined the M5 northbound at J24 Bridgwater.
It said: "Currently there are no delays on motorways in our area due to protest activity but some motorists may experience longer journey times today."
The force added: "A small number of vehicles have also set off along the A38 north from M5 J24. We understand they are travelling slowly to the Shell petrol station near Bridgwater's Express Park to conduct a protest at the forecourt."
An image posted on Facebook group Fuel Price Stand Against Tax suggests demonstrations will be held "nationwide", including in Birmingham, Cardiff, Liverpool, London and Manchester.
With most schools in England and Wales breaking up for summer this week, the RAC said an estimated 18.8 million leisure trips are planned in the UK between Friday and Monday.
That is the most since the company began tracking summer getaway numbers in 2014.
One Twitter user wrote: "I'm booked onto 8am ferry from Dover and it's total gridlock. Moved 50 metres per hour. At this rate it'll be 34 hours before I get to the port! I have a screaming toddler and three-month-old."
Another described how they have been "waiting five hours and still not in the port".
The person added: "Sat in lanes waiting to get to border control. Zero movement."
Ferry operator P&O Ferries told passengers: "There are currently queues in excess of four hours to reach the border controls.
"Our check-in remains free flowing and once you reach us, we will put you on the first available sailing. Please arrive prepared for a prolonged wait. Carry snacks and additional water with you."
Drivers were urged by Superintendent Tony Blatchford of Avon and Somerset Police to consider "alternative travel plans" due to the pump price protests.
He said: "Our protest liaison team has been engaging with the organiser so we can inform the public of the likely disruption and help to minimise it.
"Nevertheless, drivers can expect journey times will likely be longer than normal, especially on motorways, which often tend to be at their busiest at this time of year.
"We advise motorists to consider any alternative travel plans available and ensure they are suitably prepared in case they are delayed."
The A303 near Stonehenge, Wiltshire, the M4 between Cardiff and Newport in South Wales, and the M5 south of Bristol are also likely to see queuing traffic.
Travel expert Paul Charles also warned people to expect longer queues than normal at airports over the next 10 days.
But he added: "I don’t think we will have a re-run of Easter and the Jubilee weekend."
Major airports such as Heathrow are offering up to £100 extra in overtime payments per day to security staff.
It has already limited passengers to 100,000 a day and airlines like British Airways have cut more than one in ten of its flights through to October.
