All airline passengers arriving in the US from the UK are to be required to test negative for Covid-19 within 72 hours of departure amid concerns about a new coronavirus variant.
From 28 December travellers will need to provide written documentation of their test result to airlines.
Other countries have shut their borders to UK flights because of the variant.
But its rapid spread has also led to stricter rules in the UK, including a ban on overseas trips for many Britons.
And US airlines have drastically scaled back flying to the UK and Europe, after the entry of most foreign nationals was suspended at the start of the pandemic.
Health officials say there is no evidence the new variant is more deadly, or would react differently to vaccines, but it is proving to be up to 70% more transmissible.
The decision by the US Centers for Disease Control and Prevention to require testing came after New York City introduced quarantine rules for international travellers in response to the variant.
The CDC said passengers must test negative via either a PCR (polymerase chain reaction) or an antigen test.
Since Thursday, passengers travelling with Delta Air Lines and Virgin Atlantic on UK flights to the US have already been required to provide a negative test taken within 72 hours before departure.
United Airlines will introduce similar requirements for passengers travelling from the UK to the US from 28 December.
As the new variant has spread quickly in London and south-east England, rules have been tightened across the UK, meaning more than 85% of the population - 48 million people - will be in the top two tiers after 26 December.
The UK reported 39,036 Covid cases on Thursday and another 574 deaths within 28 days of a positive test.