From 04:00 GMT on 18 January, people entering the UK must show evidence of a negative coronavirus test taken up to 72 hours before boarding their inbound flight.
On arrival, most passengers must then quarantine for 10 days, or fewer if they test negative after five days.
The UK has also extended a travel ban to South America, Portugal and many African countries, amid concerns about new, potentially more infectious variants of coronavirus.
But, unlike countries such as New Zealand and Australia, the UK has never banned international travel.
The Home Office says keeping borders open in early 2020 helped businesses to maintain international connections, boosted the economy and meant people could still travel for essential reasons.
It argues that by mid-March 2020 there was “significant transmission” in the UK and any extra travel restrictions at that time would have had a “very marginal” impact.
Mark Woolhouse, professor of infectious disease epidemiology at the University of Edinburgh, says closing borders is “not a silver bullet”.
Arguments for a travel ban could be made around the new variants of coronavirus, he suggested, but it was far from certain.