By Mark Landler and Stephen Castle

LONDON — Britain was all but cut off from the rest of Europe on Monday (21), with flights and trains banned by some 40 countries and freight deliveries halted at French ports, as its neighbours tried desperately to stop a fast-spreading variant of the coronavirus from leaping across the English Channel.

The sudden disruption left Britain isolated and unnerved, its people stranded at airports or quarantined at home. It aroused fears of panic buying in British supermarkets, as a nation already rattled by a mysterious new strain of the virus now had to worry about running out of fresh food in the days before Christmas.

It all added up to a chilling preview, a mere 10 days before the deadline to negotiate a post-Brexit trade agreement between Britain and the European Union, of what a chaotic rupture between the two sides might actually look like.

For Prime Minister Boris Johnson, whose handling of the pandemic has been hampered by a reluctance to take tough measures followed by abrupt reversals in the face of alarming new evidence, the cascading events posed perhaps the gravest challenge yet to his ardently pro-Brexit government.

As he huddled in emergency meetings, Johnson was simultaneously dealing with an escalating public health crisis, deepening economic upheaval and trade talks in Brussels that could cement the break between Britain and its neighbours.

Fears of a dangerous disruption to the food supply eased somewhat over the course of the day, as French officials said they were working to devise health protocols that would allow cross-channel freight shipments to resume.

Johnson said he had telephoned President Emmanuel Macron and that the French leader told him “he was keen to sort it out in the next few hours.” Speaking at a news conference, Johnson assured Britons, “Everyone can continue to shop normally.”

Still, the multiplying problems hammered the stock market and depressed the pound. And there was an unnerving sense that Britain was entering a new, more volatile phase of the pandemic at the very moment that its relationship with its largest trading partner was being thrust into uncharted territory.

Political commentators grasped for a precedent for the chaos, with some harkening back to the turbulent events of 1978 and 1979, when nationwide strikes, compounded by harsh winter weather, led to the collapse of the Labour government and the political ascension of Margaret Thatcher.

“The government has got 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 one thing the public hates is when the government loses control.”

The trigger for the current upheaval was Johnson’s announcement Saturday (19) that he was imposing a strict lockdown on London and the southeast of England, after new data indicated that a viral mutation had turbocharged infection rates in those areas.

Scientists who briefed the press Monday estimate the variant is 50% to 70% more transmissible than the original virus (Johnson had earlier said up to 70%).  They raised the possibility that children might be more susceptible to it than to the original virus, though that could be affected by Britain’s decision to leave schools open during lockdowns, which means children are mixing more than adults.

Johnson’s move was a reversal from three days earlier, when he promised to honour his vow to ease restrictions for a few days around Christmas so that families could get together. Within hours of the announcement, thousands of people thronged railway stations and airports to try to flee London before the new rules took effect.

That, in turn, prompted countries to ban flights from Britain — a list that began with the Netherlands and Belgium and grew to encompass 17 European countries, as well as Canada, India, Russia, Jordan and Hong Kong. The United States has not yet suspended flights, though Gov. Andrew Cuomo of New York was among those urging the Trump administration to do so, and he said he had secured the agreement of British Airways, Virgin Atlantic and Delta to test all passengers bound for his state.

The European Union said it would develop a coordinated strategy for how to handle travel to and from Britain. But for now, its actions were uncoordinated, adding to the uncertainty at Heathrow, one of the main airports serving London, and other airports.

The variant has already been identified in small numbers in Denmark, the Netherlands and Australia, according to World Health Organization (WHO) officials. The Italian health ministry reported on Sunday (20) night that two people arriving in the country from Britain carried the strain.

British officials said they expected countries to ban travellers, since epidemiologists view that as necessary to break the chain of transmission across borders. But they appeared to be caught off guard by the French government’s decision to halt freight shipments, carried by truck drivers, for 48 hours.

France did not take such a step in March, when the virus first erupted in Europe, because the short crossing between the ports of Dover and Calais is a critical trade link for Britain and the continent, with thousands of trucks making the journey each day.

The restriction led to mile-long pileups on both sides of the channel, as hundreds of trucks laden with seafood and produce were stopped along the highway leading to the port of Dover. In Calais, on the French side, truckers waited for health guidance before driving their loads onto ferries to Britain. The lack of clarity left between 2,000 and 3,000 French truckers stranded on the British side.

Britain’s transport minister, Grant Shapps, said about 20% of the freight moving in and out of the country had been affected. Unaccompanied goods — such as those loaded in shipping containers — continued to be admitted into France and goods could still be driven to other countries, such as the Netherlands, from smaller ports. He also said the restrictions would not affect shipments of the coronavirus vaccine, which come from a Pfizer plant in Belgium.

Shapps described the situation as a kind of dry run for potential post-Brexit disruption, noting that the government’s contingency planning had reduced the number of trucks stuck outside the Dover port from more than 500 to about 175.

Still, the gridlock on Monday was so great that one supermarket chain warned of possible shortages of some food products ahead of Christmas, and business groups called for urgent action.

“They are carrying perishable products worth millions and the clock is ticking for that product to survive these delays,” said James Withers, chief executive at Scotland Food & Drink.

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

-New York Times