Long traffic jams built up on Saturday outside the Port of Dover, Britain's main gateway to Europe, and officials said the disruption could be worse than the logjam seen on Friday.
Travellers setting out at the start of the British 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 and the French port of Calais have "juxtaposed" border controls in which 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. "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 down here."
Roger Gough, Conservative leader of Kent County Council, said around 3,000 HGV lorries held on the M20 motorway were gradually being fed through to Dover.
Bannister said the number of French border staff at Dover had increased following Friday's disruption.
French regional prefect Georges-François Leclerc said that at midday, out of the 9,000 to 10,000 vehicles scheduled to pass on Saturday, 60% had passed without any issue.
Waiting time was about an hour and a half during the morning and was later reduced to about 45 minutes, Leclerc said on BFM TV. Asked if French customs officers were to blame for the delay, he said this was false.
"Today the situation is back to normal," Leclerc said. "The Port of Dover, which is a private port, found it easier to blame the French police (for the delays)."
British Foreign Secretary Liz Truss, who is competing to succeed Boris Johnson as prime minister, insisted on Saturday that the French were to blame.
"This is a situation that has been caused by a lack of resources at the border. That is what the French authorities need to address and that is what I'm being very clear with them about," she told reporters in Kent, southeast England, after meeting party members.
Travellers heading for the Eurotunnel service at Folkstone were also facing delays. Queues stretched two miles (3.2 km) back from the entrance.
The port delays came as some airports also struggled to recruit enough staff to manage a post-pandemic rebound in travel, leading to chaotic scenes at London airports in recent weeks. Railway travel has also been periodically disrupted this summer by labour strikes.