England considers wider lockdown to curb spread of the new variant.

As governments around the world raced to seal their borders to travelers from Britain in an attempt to contain a new variant of the coronavirus, officials in London warned that the lockdown measures in effect in the capital and southern England might need to be expanded.

The government’s chief scientific adviser, Patrick Vallance, said Monday night that the new strain was now “everywhere” across England and that cases were only expected to grow over the holiday period.

“I think it’s likely therefore that measures will need to be increased in some places in due course, not reduced,” Mr. Vallance said.

At the same time, government officials were optimistic that they could soon reach an agreement with France to end the blockade of British ports and freight going across the English Channel. French officials said they were preparing protocols that would increase testing and allow trade to start flowing again, but the details of the proposals under discussion were not made public.

With nearly every country in Europe now banning travel to and from Britain, European Union officials struggled to establish guidelines for member nations that could bring a semblance of order to the chaotic cascade of border closures announced over the past 48 hours.

The World Health Organization sought to temper the growing sense of alarm over the new variant and the danger it poses. “There is zero evidence at this point that there is any increase in the severity associated with this disease,” Mike Ryan, the W.H.O. emergency director, said at a virtual news conference. “Clearly there is work ongoing to look at transmission and the increased rates of transmission and how much of that is attributable to this particular variant.”

Dr. Ugur Sahin, the co-founder of BioNTech, which developed the first vaccine approved in the West to combat the coronavirus, said that there was no evidence at the moment to suggest that the current vaccine would have to be adapted to be effective against the new variant.

But, he cautioned, it would be two weeks before full results from laboratory studies would allow for a fuller understanding of how the mutations might impact the effectiveness of the vaccine.

“We believe that there is no reason to be concerned until we get the data,” he said.

If an adapted vaccine were necessary, it could be ready within six weeks, Dr. Sahin told a news conference on Tuesday. But it would require additional approval from regulators, which could increase the wait-time, he said.

While scientists around the world worked to better understand the dangers of the variant, the measures being put in place to try and contain it were already being felt across Britain. Millions of people forced to abruptly cancel Christmas plans are now also facing the prospect of a disruption to basic services unlike anything experienced so far in the pandemic. Even the Royal Mail was forced to temporarily suspend all services to Europe, with the exception of Ireland, because of the “current restrictions around air, road, ferry and train movements.”