The precautions raised fears of food shortages in Britain if the restrictions drag on.

After a conversation with French President Emmanuel Macron, British Prime Minister Boris Johnson said he understood the reason for the new measures and expressed hope for a swift resumption in the free flow of traffic between the U.K. and France, perhaps within a few hours.

He said officials from both countries were working “to unblock the flow of trade as fast as possible.” Macron said earlier that France was looking at establishing systematic testing of people for the virus on arrival.

Over the weekend, Johnson imposed strict lockdown measures in London and neighboring areas where Health Secretary Matt Hancock said the new strain is “out of control.” Experts said the early evidence indicates the strain is not more lethal, and they expressed confidence that the vaccines now being rolled out would still be effective against it.

After France announced Sunday night that it was closing its borders to trucks from Britain, hundreds of vehicles, mainly operated by drivers from continental Europe, became stranded outside the English Channel port of Dover. The lines shrank over the course of the day from 500 trucks to about 175 before rising again in the evening to 945, authorities said. Vehicles are being redirected to the disused Manston Airport nearby, which is being prepared to accommodate up to 4,000 trucks.

Around 10,000 trucks pass through the port of Dover every day, accounting for about 20% of the country’s trade in goods.

Meanwhile, the European Union gave the go-ahead to the coronavirus vaccine developed by Pfizer and BioNTech, setting the stage for the first COVID-19 shots across the 27-nation bloc to begin just after Christmas.

“This is a very good way to end this difficult year and to finally start turning the page on COVID-19,” said European Commission President Ursula von der Leyen.

The approval came just hours after the EU’s drug regulatory agency said the vaccine meets safety and quality standards. It is already being dispensed in Britain and the U.S.

The virus is blamed for 1.7 million deaths worldwide, including about 68,000 in Britain, the second-highest death toll in Europe, behind Italy’s 69,000.

While the French ban does not prevent trucks from entering Britain, the move stoked worries about shortages at a time of year when the UK produces very little of its food and relies heavily on produce delivered from Europe by truck.

Many trucks that carry cargo from the U.K. to the continent return laden with goods for Britain’s use. The fear is that the ban will lead to a drop in such deliveries. Also, some drivers or their employers might decide against entering Britain for fear they won’t be able to get back home.

Supermarket chain Sainsbury’s warned that some products, such as lettuce, cauliflower, broccoli and citrus fruits, could soon be in short supply if the crisis not resolved in the coming days.

Canada, India, France, Germany, Italy, Spain and Sweden were among the countries that suspended flights from Britain for various lengths of time.

In the United States, New York Gov. Andrew Cuomo, who has been calling on the U.S. to halt flights as well, said he asked airlines flying into his state from the U.K. to make all passengers take a coronavirus test before boarding. He said at least one carrier, British Airways, agreed to do so.

At Germany’s Berlin and Frankfurt airports, passengers arriving from Britain had to spend Sunday night in the terminals as they awaited test results.

Eurotunnel, the rail operator that carries passengers and freight between Britain and mainland Europe, also suspended service out of the U.K.

Sophie Taxil, a Frenchwoman who lives in London, caught a train back home from Paris and urged everyone in Britain to follow the rules.

“I live there and I need to go back there; my family is there,” she said. “I think that phlegmatic British nature suits these COVID times: Stay calm and carry on, fair play.”

Over the weekend, Johnson said early indications are that the variant is 70% more transmissible and is driving the rapid spread of infections in the capital and surrounding areas.

As a result, he scrapped a planned relaxation of rules over Christmastime for millions of people and imposed other tough new restrictions in the affected zone. No indoor mixing of households will be allowed, and only essential travel will be permitted. Stores selling nonessential goods were ordered closed, putting a crimp in Christmas shopping.

The European Centre for Disease Prevention and Control said that while preliminary analysis suggests the new variant is “significantly more transmissible,” there is no indication that infections are more severe. Still, experts have stressed that more infections will lead to more hospitalizations and deaths.

The Stockholm-based agency said a few cases of the variant have been reported by Iceland, Denmark and the Netherlands. It also cited news reports of cases in Belgium and Italy.

With the rollout of coronavirus vaccines set to pick up speed in early 2021, countries are clearly worried about the new variant.

“I think the vaccine will be fine,” said Dr. Julian Tang, a clinical virologist at the University of Leicester. But the new strain “is spreading so fast we may not be able to reach the vulnerable population, the elderly, in time with the vaccine before the virus does.”

The chaos at the border comes at a time of huge uncertainty for Britain, less than two weeks before it completes its exit from the EU and frees itself from the bloc’s rules. Talks on a post-Brexit trade relationship between the two sides are deadlocked.

The retail industry played down fears of food shortages in the short term but warned of problems if the travel bans last for a while and if Britain and the EU fail to reach a trade deal. Trade association Logistics U.K. urged people to stay calm and resist panic-buying in supermarkets.

“If freight gets moving again today, then the overall impact on fresh produce arriving to supermarkets should be fairly minimal,” said Kevin Green, the association’s director of marketing and communications.