LONDON — France agreed Tuesday to reopen its borders to travelers from Britain and get trade flowing again, but it may take days to clear out the thousands of cargo trucks snarled while a travel ban was in place, prompted by fears of a fast-spreading coronavirus mutation in England.

More than 50 countries have enacted restrictions on arrivals from Britain, disrupting passenger air service between the United Kingdom and the rest of the world. But France’s ban was particularly disruptive, halting transit along one of the most crucial trade routes in Europe.

The French government on Tuesday announced a reopening, starting Wednesday, for European Union citizens and residents if they provide a negative coronavirus test from the previous 72 hours. Truck drivers of all nationalities will be permitted entry as long as they, too, can provide evidence of a test if asked.

After France and Britain haggled into the night over what sort of test would be acceptable, the two sides agreed to allow lateral flow tests, which provide results in about 30 minutes, along with the longer-turnaround PCR (polymerase chain reaction) tests.

British Transport Secretary Grant Shapps said stranded truck drivers would start getting coronavirus tests Wednesday, Reuters reported, so they could start to cross over to France.

“We’ll be making sure that tomorrow we’re out there, providing tests,” Shapps said. “This will take two or three days for things to be cleared.”

The leader of the Kent County Council in southeast England told the BBC on Tuesday that 3,000 trucks were waiting to cross the English Channel. On Monday, British government officials claimed there were only 170.

Drivers honked their horns in frustration as some prepared to sleep for a third night in their cabs.

Prime Minister Boris Johnson has promised his country that a newly sovereign, free-trading, swashbuckling “Global Britain” will soar after Brexit, coming in less than nine days.

But with no free-trade deal signed between Britain and the E.U., and the gateway to continental Europe blocked this week, Britons found themselves more isolated than ever.

Adding to the sense of loneliness, after saying that it would be “inhumane” to cancel Christmas gatherings of friends and family, Johnson felt compelled to do just that on Sunday, based on advice from his public health advisers, who see cases of the coronavirus soaring here.

More than 18 million Britons are now in “Tier 4” shutdown, with all nonessential shops, pubs, restaurants, gyms, hair salons, theaters and toy stores closed.

Anthony S. Fauci, the top U.S. infectious-disease official, told the BBC on Tuesday he thought a total U.S. ban on arrivals from Britain “might be a bit of an overreaction.”

The European Commission on Tuesday sought to promote a more coordinated approach after different levels of restrictions were announced haphazardly over the previous days.

It urged all 27 member states to end bans on flights and trains from Britain and to reopen freight routes. “All nonessential travel to and from the UK should be discouraged,” the E.U.’s executive arm said in a statement, but “flight and train bans should be discontinued given the need to ensure essential travel and avoid supply chain disruptions,” including the delivery of coronavirus vaccines.

Despite the commission’s recommendation, several countries continued to head in the opposite direction Tuesday. Hungary banned passenger planes from the United Kingdom until early February, while Germany and Ireland extended their entry restrictions.

The Netherlands announced that its ban on travelers from Britain and South Africa would be replaced by a requirement to show evidence of a recent negative PCR test. The Dutch government also urged passengers to quarantine for 10 days after arrival.

The tussle over travel and trade came as scientists in Britain and around the world scrambled to assess the impact of the new mutation of the coronavirus, first spotted in England.

Sharon Peacock, the director of the U.K. consortium tracking mutations of the virus, said Tuesday that “we have no evidence that the vaccine is in any way compromised by this new variant.”

Judith Breuer, professor of virology at University College London, said the observed rise in the prevalence of the variant strongly suggests the new mutation is driving increased transmission.

Breuer said that in coming weeks, researchers will be able to see whether the greater spread of the variant leads to more-serious illness, an observation that might be captured by a spike in hospitalizations and deaths. So far, she said, there has been no evidence to show the mutation is more deadly.

BioNTech chief executive Ugur Sahin said Tuesday that the vaccine his company developed in partnership with Pfizer is likely to be effective against the new variant identified in Britain but that a new version of his vaccine could be developed within six weeks if necessary.

From a technical perspective, tweaking the vaccine would simply be a matter of replacing one mutation with another while the “messenger” molecule remained the same.

Whether regulators would be willing to quickly approve a slightly modified version of the vaccine that has been cleared for distribution in the United States, Britain and the E.U. is another story, Sahin told reporters at a news conference.

“The likelihood that our vaccine will work is relatively high,” Sahin said, noting that 99 percent of proteins found in the new mutation are the same as in other strains.

The French travel restrictions imposed Sunday night, with an initial 48-hour time frame, did not ban freight coming into Britain, but flows in that direction were nevertheless hampered. Few companies appeared willing to risk sending their drivers over only to get stuck in the United Kingdom for Christmas.

Even after France agreed to reopen its borders, it remained to be seen whether the new testing requirements would deter truckers carrying goods destined for British shores.

Noack reported from Berlin. Ariès reported from Brussels.