(CN) — A potentially dangerous mutation of the novel coronavirus found to be spreading rapidly in the United Kingdom is causing alarm across Europe and prompting countries to close off travel with Britain.

On Saturday, the U.K. imposed strict lockdown measures to stop the spread of a new variant of the virus that British Prime Minister Boris Johnson said is much more transmissible, though he said there was no evidence it was more lethal or more resistant to vaccines.

British officials said this new variant is rapidly proliferating in London and elsewhere in southern England. About 60% of London’s new cases are linked to the new variant, which Johnson said was 70% more transmissible.

By Monday, the European Union largely sealed off travel to the U.K. in an effort to clamp down on the spread of the new variant, whose appearance in Europe casts the hard-hit continent into new turmoil. Other non-EU countries were also shutting down travel to the U.K. On Monday, New York Governor Andrew Cuomo called on the U.S. to ban U.K. flights.

European financial markets suffered losses on Monday and the threat of this new strain cast shadows over a Monday approval by EU medical authorities of the Pfizer-BioNTech vaccine against Covid-19, the disease caused by the coronavirus.

The threat caused by the coronavirus variant remains unclear, though if it spreads more easily that poses a clear danger. Scientists are divided over whether it is more transmissible, according to media reports.  

On Friday, World Health Organization experts said there was no evidence the new strain was more dangerous nor that it would render vaccines less effective. Since the novel coronavirus was first discovered in China, it has undergone many mutations but WHO experts say such changes are to be expected and that they have not been so extreme as to make vaccines being developed against it ineffective.

British authorities say more than 1,100 new cases have been linked to the variant. The new strain has been found in Denmark, Belgium, the Netherlands, Italy, Australia and South Africa. The European Center for Disease Prevention and Control said it appeared the variant has been circulating since at least November.

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