Airline passengers flying to the US from the UK will have to produce a negative Covid-19 test from the three days before travel as authorities there try to prevent the new coronavirus variant gaining a toehold in the country.
Travellers will be banned from boarding aircraft if they cannot provide written documentation of a lab-based test to the airline, the Centres for Disease Control and Prevention said in a Christmas Day statement.
The new rules will be applied from 28 December onwards, it added.
The restrictions follow a week in which countries across the globe have shut their borders to UK travellers amid fears over the mutated variant, which is thought to have emerged in Kent and which is believed to be far more infectious than previous variants.
China, Canada and much of Europe have all banned all flights from Britain, while parts of Kent have been turned into giant lorry parks after France refused admission for truck drivers until they had taken tests.
In the US itself, New York City authorities had already introduced quarantine rules for international travellers. All visitors are ordered to stay at the address they provided on arrival for 14 days.
In the UK, meanwhile, some 39,036 new coronavirus cases were reported on Christmas Eve with another 574 deaths.
But there are growing concerns that even those numbers may soon be dwarfed with the new variant predicted to result in an explosion of cases all across the UK within the coming days and weeks.