Thousands of holidaymakers have seen their plans thrown into chaos after UK quarantine measures were imposed on France from Saturday.
The 14-day isolation requirement begins at 04:00 BST and also applies to people arriving from the Netherlands, Monaco, Malta, Turks and Caicos, and Aruba.
It comes after France’s prime minister acknowledged infection numbers were going “the wrong way”.
France warned it would take “reciprocal measures”.
Clement Beaune, France’s secretary of state for European affairs, tweeted that the UK’s decision was a matter of “regret” for the French.
Transport Secretary Grant Shapps said the decision was triggered when the rate of infection in the affected countries exceeded 20 cases per 100,000 people.
“We’ve worked so hard in this country to get our level of infections down, the last thing we want do is to have people returning and bringing the infection with them. It’s to protect everybody,” he told BBC Breakfast.
There are about 160,000 British holidaymakers currently in France, he said. The deadline is expected to induce a rush to ports and airports, with thousands of tourists desperate to avoid quarantine.
The Eurotunnel website is reportedly struggling to deal with the volume of inquiries.
Mariana Fabricante, who is trying to return from the mountain resort of Tignes with her family, said: “Every time I try to change the ticket, the website is busy. People would be able to make informed decisions if they had been told in advance. It’s annoying and frustrating.”
John Keefe, director of public affairs at Getlink, which operates the Channel Tunnel, told BBC’s Newsnight programme that the trains were “already pretty much fully booked” on Friday.
He said: “We just haven’t got the space to take everybody who might suddenly want to come up to the coast.”
Urging travellers to check online if there is space for them before heading to the terminal, he said people should understand “that it’s not going to be easy to get back” to the UK.
Airlines UK described it as “another devastating blow to the travel industry already reeling from the worst crisis in its history”.
The UK’s ambassador to France, Lord Llewellyn, acknowledged that the new quarantine rule would be “unwelcome news” for Britons in the country, but stressed that people could continue with their holidays as long as they follow safety precautions and self-isolate on their return.