LONDON — The British government scrambled Monday to dampen the impact of travel entry bans from more than 40 countries prompted by fear of a new coronavirus mutation believed to have originated in England.

Health officials said they do not believe the mutation is more deadly or vaccine resistant, but evidence that it spreads much more easily has alarmed governments in Europe and around the world.

Scientists advising the British government estimated that the new variant was 50 percent more transmissible.

As country after country shut their borders to people traveling from Britain, the greatest travel disruption emerged along both sides of the English Channel, after France imposed a far more comprehensive ban than the border closures during the first wave of the virus in spring.

The short passage from Britain to France across the channel is one of the most important transport corridors in Europe. On Monday, hundreds of trucks backed up for miles, prompting concern that food and other time-sensitive cargo might end up rotting on the roadside.

British supermarket chain Sainsbury’s warned: “If nothing changes, we will start to see gaps over the coming days on lettuce, some salad leaves, cauliflowers, broccoli and citrus fruit — all of which are imported from the continent at this time of year.”

The hashtag #lettuce trended on Twitter in Britain, as panicked shoppers tried to stock up on supplies for Christmas and beyond.

In a news conference at Downing Street on Monday evening, Prime Minister Boris Johnson played down the impact of border closures and batted away questions about the need for a national lockdown, while emphasizing progress on vaccines. More than 500,000 people in Britain have gotten an initial Pfizer-BioNTech shot.

Johnson said he understood the fears of other countries about the new virus strain, but was talking to French President Emmanuel Macron — who is self-isolating after coming down with covid-19 last week — about getting the ports moving again.

Meanwhile, Johnson reported, the line of trucks idling at the Dover port had been reduced from 500 to 170. And he said France’s travel ban affected only 20 percent of the trade going in and out of Britain — freight carried by trucks with drivers, who travel by ferry or tunnel.

“Which means the vast majority of food, medicines and other supplies are coming and going as normal,” Johnson said, adding that British supermarket supply chains are “strong and robust.”

British scientists advising the government said Monday that this coronavirus mutation first arose in England in September, but it wasn’t until December that researchers saw how quickly it began to dominate. Today, 80 percent of newly diagnosed cases in London, for example, were likely caused by the new mutation.

A review of the latest data underscored “high confidence” that the new strain has a transmission advantage over earlier versions of the coronavirus seen in Britain, said Peter Horby, professor of emerging infectious diseases at the University of Oxford.

The scientists said that in nasal and throat swabs taken from patients, there appeared to be more virus particles present, compared to earlier versions.

Researchers on Monday also flagged early evidence that children appeared to be more susceptible to the new strain, although they cautioned it did not make the children more likely to have symptoms or become sick.

The new strain may make children “as equally susceptible as adults,” said Wendy Barclay, head of the infectious-disease department at Imperial College London.

Adam Finn, a professor of pediatrics at the University of Bristol, said Monday that the new strain was being tested to see if it might be more resistant to vaccines. “It is a matter of immediate interest,” he said, adding that predictions are it will “have either no effect or a minor effect” on the efficacy of vaccines.

In the interim, scientists said Britain’s vaccine rollout should continue as quickly as possible.

British officials were left scrambling Dec. 21 as much of Europe closed its borders to the U.K. as it battles a new strain of the coronavirus. (Reuters)

The World Health Organization sought to temper panic. Chief scientist Soumya Swaminathan said the coronavirus is mutating at a much slower rate than the seasonal flu. “And so far, even though we’ve seen a number of changes and a number of mutations, none has made a significant impact on either the susceptibility of the virus to any of the currently used therapeutics, drugs or the vaccines under development, and one hopes that that will continue to be the case,” she said.

Tobias Kurth, director of the Institute of Public Health at Berlin’s Charité University Hospital, said the decision by numerous countries to “pull the emergency brakes” and suspend travel with Britain is “understandable.”

But Kurth cautioned that the mutation is “certainly already in continental Europe, and likely in Germany.”

“We won’t be able to stop it,” even though travel restrictions may slow the spread of the mutation, he said.

French Health Minister Olivier Véran acknowledged Monday morning that the new variant may already be in France. Italy, the Netherlands and Denmark said they have identified the mutation among recently discovered coronavirus cases in their countries.

Mutations of the virus that share traits with the British variant have also been detected in South Africa and are responsible for an increases in infection there.

In Britain, officials on Monday announced 33,364 new coronavirus cases and 215 deaths.

With cases soaring, some stoked by the new variant, Johnson ordered London and parts of the southeast England into Tier 4 lockdown over the weekend, telling 18 million people to “stay at home” and only venture out to shop for food and medicine, attend medical appointments or do outdoor exercise.

The travel bans compounded the upheaval. They come less than two weeks before Britain is set to cut its last membership ties with the European Union. The two sides have yet to agree on a post-Brexit trade deal, and the disruptions on Monday provided a preview of what could happen if Britain crashes out of the bloc without one.

Johnson tried to suggest that anticipation of a possible no-deal Brexit offered an advantage when faced with pandemic-related border closures. “The government has been preparing for a long time for exactly this kind of event,” he said.

British Transport Minister Grant Shapps said the government was providing portable toilets for stranded lorry drivers and rerouting trucks at Britain’s southern port, where tens of thousands of trucks normally converge every day to board ferries or travel through the Eurotunnel to France. Authorities made contingency plans to deliver medicines by military helicopter if necessary.

Earlier Monday, French Transport Minister Jean-Baptiste Djebbari had tweeted that in the coming hours, in coordination with other European nations, the country would “put in place a robust sanitary protocol to allow traffic flows from the United Kingdom to resume.” But no announcement had been made by evening.

Even though the European travel restrictions do not ban trucks from entering the United Kingdom, industry representatives cautioned that few companies would be willing to take the risk of then becoming stranded there, meaning trade was impacted in both directions.

“No driver wants to deliver to the U.K. now, so the U.K. is going to see its freight supply dry up,” said Vanessa Ibarlucea, a spokeswoman for the French road haulage federation, according to Reuters.

Beyond the ports, many passengers were left stranded, as more countries canceled flights to and from the United Kingdom.

Beth Gabriel Ware, a British citizen who lives in Turkey, found herself stuck at her parents’ home in Kent after the Turkish government banned flights from Britain on Sunday. She had surprised her family with a visit after they had been apart for 10 months.

“I will be sleeping on the couch for the foreseeable future,” said Ware, 23.

Hind Mrabet, 21, who planned to move from Britain to Paris for school at the end of this week, now does not know when she will be able to cross into France.

“They seem to be making last-minute decisions that leave people in panic,” she said of the British government.

Noack reported from Berlin. Ruby Mellen in Washington contributed to this report.