Travellers setting out at the start of Britain's school summer holidays, as well as the usual flow of goods lorries, have faced long delays because of slow border checks.
The Kent Resilience Forum, which brings together local councils and emergency services, declared a "major incident", while Port of Dover CEO Doug Bannister said travellers could face delays of up to six hours on Saturday.
The UK government has blamed the problems on a lack of French border force staff, while France says more checks are needed because Britain is no longer a member of the European Union.
Dover, which handles 12 million customers a year, and its French 'sister port' of Calais have what is known as juxtaposed border controls, where French authorities check passports on British soil before departure, and vice versa in France.
"We were expecting that today was going to be a busier day than yesterday," Bannister told BBC radio on Saturday.
"Yesterday we processed about eight-and-a-half thousand cars going out, today we were predicted to be around 10,000 - so it is going to be a very busy day."
Roger Gough, the Conservative leader of Kent County Council, said around 3000 heavy goods vehicles held on the M20 motorway were gradually being fed through to Dover.
Bannister said the number of French border staff at the port had increased following Friday's disruption.
British Foreign Secretary Liz Truss, who is competing with former treasurer Rishi Sunak to succeed Boris Johnson as prime minister, said on Friday the "awful situation" was 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," she said in a statement.
Travellers heading for the alternative Eurotunnel service at nearby Folkestone were also facing delays. Queues stretched more than three kilometres back from the entrance.
The major port delays come as airports struggle to recruit enough staff to manage the post-pandemic rebound in travel, leading to chaotic scenes at London airports in recent weeks.
Rail travel has also been periodically disrupted by strikes during the UK's busy summer.
