Passengers are warned to allow three to four hours to clear roads and security checks at the port.
Travellers heading to France are facing delays of several hours amid continuing gridlock on roads near Dover and the Eurotunnel terminal in Folkestone.
More than 17,000 passengers had left Dover on ferries by lunchtime on one of the busiest weekends of the year.
P&O Ferries warned customers to allow three to four hours to clear local roads and security checks on Saturday.
Passengers using Eurotunnel also faced waits of several hours on the journey to the Folkestone terminal.
This weekend is one of the busiest for UK overseas travel as most schools in England and Wales have broken up for the summer.
Around 10,000 cars were expected to pass through Dover on Saturday, on top of 8,500 on Friday.
Officials in Kent declared a major incident on Friday due to the traffic. People trying to get away via Dover were held up in lengthy queues, as routes to the ferry terminal were in gridlock.
Traffic queues four miles long also formed on the eastbound M2.
Some 3,000 lorries are also waiting to cross the Channel. They are currently parked on the M20 outside Dover, while holiday traffic is prioritised.
UK officials claimed that a lack of French border staff had caused the delays, but this was rejected by Calais MP Pierre-Henri Dumont.
He said additional checks post-Brexit and a lack of capacity at the port were behind the problem.
Passengers using Eurotunnel were asked to arrive two hours early by the operator and faced gridlock on the routes to the terminal in Folkestone on Saturday.
John Keefe, director of public affairs for Eurotunnel, said traffic disruption had been caused by an accident on the M20 on Friday, and stretches of the road being used solely for freight traffic, with holidaymakers diverted on to A-roads.
Backlog is starting to clear
By Dan Johnson, BBC News
We’re told that things are flowing a bit more freely and that they are starting to eat into that backlog that built up yesterday.
There are a lot of cars queuing here at the port at the moment. But in a sense this is the good queue. This is the queue you want to be part of.
The problem is that thousands of people are stuck on the roads miles out from Dover trying to get across Kent to the port.
They’ve been held back by the police operations that have been in place to try to manage the traffic, to try to stop Dover itself getting completely snarled up.
And that does seem to have been a bit more successful today, but drivers are still being warned that they do face big delays.
The Port of Dover and the UK government blamed the delays on France, saying they did not have enough border staff.
French authorities said an “unforeseeable technical incident” in the Channel Tunnel meant French border police were delayed into Dover – but Eurotunnel rejected this explanation.
Foreign Secretary Liz Truss said the delays were “unacceptable” and the situation was “entirely avoidable”, calling on France to build up capacity at the border.
‘How are we feeling? Sad and frustrated’
If you’re a parent, you probably know how challenging long car trips with a young child can be.
Dave Harvey and his family faced lengthy delays on the roads on their way to the Eurostar terminal at Folkestone.
He said they had queued for three hours to get through, and did not understand why the M20 had been closed.
“[It’s a] bit of a joke really,” Mr Harvey said.
Asked how he felt at the start of his family summer holiday, he replied: “Frustrated. Especially with the young one. Sad and frustrated.”
Dover MP Natalie Elphicke said Saturday was another “really busy and difficult day”, and also pointed the finger at the French, saying they had “let us down badly” on Friday.
But Mr Dumont said problems at the port would happen again because more checks are needed since Brexit.
He also told BBC News the Port of Dover was “too small” and there were too few kiosks due to lack of space.
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