"There is zero evidence that the new coronavirus variant increases severity of the disease," World Health Organization's Health Emergencies Program chief Mike Ryan said in a statement late Monday.
Countries around the world closed their borders to Britain earlier on the same day in a race to contain a fast-spreading variant of the coronavirus as the European Union neared approval of its first jab for the bloc's vaccination campaign.
Dozens of countries from India to Argentina suspended flights from the UK in fear of the new virus strain, delivering a bleak reminder that the pandemic is far from under control a year after it began even as vaccinations bring a rush of optimism.
European stocks, oil prices and the British pound all plummeted over the developments.
Thousands of people are still dying daily from the virus that has claimed at least 1.69 million lives over the past year since it first emerged in China.
While experts say there is no evidence the UK variant of the virus -- one of several mutations -- is more lethal or will affect the impact of vaccines, it may be up to 70 percent more transmissible, according to early data.
The rapid response of flight cancellations upended travel plans around the world just as families were preparing to unite for Christmas.
"Please help us leave!" said one British traveller who was among dozens held overnight in German airports, unable to leave until virus tests come back negative.
Others are now moored in the UK for the foreseeable future.
Beth Gabriel Ware, a Briton who lives in Turkey, flew home earlier this month to surprise her parents in Kent, south-east England -- but is now stuck.
"I hadn't been in the UK for about 10 months, because I was so scared of this happening," she lamented.