UK visitors are now able to travel to the US for the first time in nearly two years.
The border reopened at 05:01 GMT and the first flights take off from Heathrow at 08:30 – but all UK visitors over 18 will have to provide proof of vaccination to enter the US.
American travellers have been able to travel to the UK since 28 July.
Transport Secretary Grant Shapps has called this a “significant moment” for UK-US travel.
Transatlantic flights, he added, are “at the heart of UK aviation”.
In addition to the UK, the travel ban is being lifted for people from Brazil, China, India, Ireland, South Africa, Iran and the Schengen countries – a group of 26 European nations.