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.