LONDON — Britain was all but cut off from the rest of Europe on Monday, with flights and trains banned by some 40 countries and freight deliveries halted at French ports, as its neighbors 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 neighbors.

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.”

The trigger for the current upheaval was Johnson’s announcement Saturday 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’s move was a reversal from three days earlier, when he promised to honor 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.