LONDON – The UK was virtually cut off from the rest of Europe on Monday as flights and trains were banned from around 40 countries and freight deliveries in French ports ceased as its neighbors desperately tried to get the quick jump of a fast-spreading variant of the coronavirus prevent crossing the English Channel.

The sudden disruption left Britain isolated and unsettled, its people stranded at airports or quarantined at home. It sparked fear of panic buying in UK supermarkets as a nation already shaken by a mysterious new strain of the virus had to worry about running out of fresh groceries in the days leading up to Christmas.

It all adds up to a terrifying preview, just 10 days before the deadline for negotiating a post-Brexit trade agreement between the UK and the European Union, of what a chaotic break between the two sides could actually look like.

For Prime Minister Boris Johnson, whose handling of the pandemic has been hampered by reluctance to take tough action followed by abrupt reversals in the face of alarming new evidence, the cascading events represented perhaps the greatest challenge to his passionate pro-Brexit administration.

As he pushed himself into emergency meetings, Mr Johnson was also addressing an escalating public health crisis that deepened economic upheaval and trade talks in Brussels that could cement the rift between Britain and its neighbors.

Fears of a dangerous disruption to food supplies eased somewhat as the day wore on as French officials worked to draft health protocols that would allow cross-channel freight to resume.

Mr Johnson said he called President Emmanuel Macron and that the French leader told him “he wanted to sort it out in the next few hours”. At a press conference, Mr Johnson assured the British: “Anyone can go on shopping normally.”

Still, the multiplication problems hammered the stock market and depressed the pound. And there was a worrying feeling that the UK was entering a new, more volatile phase of the pandemic as its relationship with its largest trading partner was being pushed onto uncharted territory.

Political commentators sought a precedent for chaos, some dating back to the tumultuous events of 1978 and 1979 when nationwide strikes compounded by harsh winter weather led to the collapse of the Labor government and the rise of Margaret Thatcher.

“The government has to do something to get things under control,” said Jonathan Powell, who served as chief of staff to former Prime Minister Tony Blair, when a series of strikes in 2000 caused a politically damaging fuel crisis. “The only thing the public hates is when the government loses control.”

The trigger for the current upheaval was Mr Johnson’s announcement on Saturday to strictly lock down London and the south-east of England after new data indicated that a virus mutation had boosted infection rates in those areas.

Scientists, who informed the press on Monday, estimate that the variant is 50 to 70 percent more transmissible than the original virus. (Mr Johnson had previously said up to 70 percent.) You raised the possibility that children are more susceptible to it than the original virus, although that could be influenced by the UK’s decision to keep schools open during the lockdown, which means that children are more mingle than adults.

Mr. Johnson’s move was a reversal from three days earlier when he promised to keep his vow to relax restrictions for a few days around Christmas so families could get together. Within hours of the announcement, thousands of people were rushing to train stations and airports to try to escape London before the new rules took effect.

This, in turn, prompted countries to ban flights from the UK – a list that started with the Netherlands and Belgium and included 17 European countries, plus Canada, India, Russia, Jordan and Hong Kong. The United States has not suspended flights yet, although New York Governor Andrew M. Cuomo was among those urging the Trump administration to do so.

The European Union announced that it would develop a coordinated strategy for dealing with travel to and from the UK. However, at the moment, actions have not been coordinated, adding to uncertainty at Heathrow, one of London’s main airports, and other airports.

According to representatives of the World Health Organization, the variant has already been identified in Denmark, the Netherlands and Australia in small numbers. The Italian Ministry of Health reported Sunday evening that two people who came to the country from the UK were bearing the strain.

British officials said they expected countries to ban travelers as epidemiologists see it as necessary to break the chain of cross-border transmission. However, the French government’s decision to suspend trucker haulage for 48 hours seemed to surprise them.

Updated

Apr. 21, 2020, 3:20 pm ET

France did not take such a step in March when the virus first broke out in Europe, as the short crossing between the ports of Dover and Calais is a vital trade link for the UK and the continent, with thousands of trucks traveling daily.

The restriction resulted in miles of piles on either side of the canal as hundreds of trucks loaded with seafood and produce stopped along the freeway to the port of Dover. In Calais on the French side, truckers waited for health advice before putting their cargo on ferries to Great Britain. The ambiguity left between 2,000 and 3,000 French truckers stuck on the British side.

UK Transport Secretary Grant Shapps said around 20 percent of cargo in and out of the country would be affected. Unaccompanied goods – such as those loaded in shipping containers – continued to be allowed into France, and goods could still be brought from smaller ports to other countries such as the Netherlands. He also said the restrictions would not affect shipping of the coronavirus vaccine, which comes from a Pfizer plant in Belgium.

Describing the situation as a kind of dry run for possible disruptions after Brexit, Mr Shapps noted that the government’s contingency planning had reduced the number of trucks stuck outside the port of Dover from more than 500 to around 175.

Nevertheless, the standstill on Monday was so great that a supermarket chain warned of possible bottlenecks in some food before Christmas and corporate groups urgently demanded action.

“They carry millions of perishable products and the clock is ticking for that product to weather those delays,” said James Withers, managing director of Scotland Food & Drink.

He estimated that Scotland would typically deliver £ 5 million worth of groceries, about $ 6.7 million, to France every day this week. The UK sends more seafood to the European Union than it imports, especially stocks of salmon, lobster and langoustines.

The clutter, Withers said, should lead the government to rethink what will happen at the end of the Brexit transition period on December 31.

“The UK government needs to recognize that we are in the middle of a perfect storm and that it is totally unacceptable to risk further disruption and financial damage to businesses in just 10 days,” he said.

Supermarket chains said food supplies were already in hand for Christmas, but if the travel suspension lasted longer, there would be a shortage of items like lettuce, vegetables, cauliflower, broccoli and citrus fruits. About a quarter of the food consumed in the UK is imported from the European Union.

“Continental truckers won’t want to travel here if they’re really scared of getting stranded,” said Ian Wright, executive director of the Food and Drink Federation.

The closure of ports also disrupts parcel delivery. On Monday, Deutsche Post DHL announced that deliveries to the UK would also be discontinued as more and more countries were imposing travel bans on the UK.

With years of experience dealing with disruptions, the Port of Dover usually manages to clear traffic jams quickly once ferry service resumes. However, the situation remained fluid and additional health checks could result in further delays.

The impact of the chaos on the UK trade negotiations with Brussels was not clear. The negotiators failed to reach an agreement by Sunday evening that the European Parliament had set as the deadline for ratifying the agreement so that it could enter into force on January 1st.

But talks in Brussels resumed Monday, with both sides saying a deal could be finalized even at this late stage. The negotiators appeared to have made some progress on the most contentious issue – European access to UK fishing grounds – although there was no sign of an imminent breakthrough.

Some analysts interpreted France’s decision to suspend freight operations as a tough tactic to remind Britain of the costs of not closing a trade deal. Others said they were impressed with the lack of communication between the UK and its European neighbors after scientists identified the variant in the UK a few weeks ago.

“If the Johnson administration had thought this through and actually talked to its partners, this could have been handled better,” said Anand Menon, professor of European politics at King’s College London. “The information exchange seems to have failed.”

Mr Menon said the prime minister’s last minute style and lack of consultation would make it politically difficult for him to both sell a trade deal and convince lawmakers of the need for further lockdowns.

After Mr Johnson announced his final lockdown on Saturday, some of his Conservative Party’s lawmakers called for a Parliament recall, but there are currently no plans for one.

“The idea that this government treats parliament with contempt has taken root,” Menon said. “It makes the political costs of it harder to bear.”

While the threat of a rapidly expanding variant would under no circumstances have resulted in a ban on travel, analysts said Brexit made it politically easier for European governments to isolate the UK.

Mr. Powell, former aide to Tony Blair, recalled an apocryphal headline: “Fog in Channel; Continent Cut Off ”- this is often used to describe an inward looking UK and its independent relationship with the rest of Europe.

“Little England has always wanted to cut the continent off,” Powell said. “You finally did it.”

Esha Nelson contributed to the coverage