Most people arriving in Britain from Monday will have to self-isolate for two weeks under a new coronavirus restriction that has been roundly condemned by the ailing aviation sector.
The measure, which applies to both residents and visitors with some exceptions, aims to prevent a second wave of contagion from abroad.
But critics question why Britain - where the number of new COVID-19 infections per 100,000 people in the past fortnight was the second-highest in Europe - is inflicting more pain on hotels and airlines by reducing travel from countries with fewer virus cases.
British Airways and budget carriers EasyJet and Ryanair have launched joint legal proceedings against the government over what they called a "disproportionate and unfair" step.
Health Secretary Matt Hancock said the new rules made sense because "the proportion of infections that come from abroad increases" as Britain's own caseload drops.
"We've got to take an approach that starts with caution," he told Sky News.
To enter Britain by plane, train, road or sea, travellers must provide details of their journey and the address where they will self-isolate.
How the quarantine will be implemented differs between Britain's devolved nations, and the measures will be assessed every three weeks.
Exemptions are being made in several cases - including for truck drivers, "essential" healthcare workers and people travelling from Ireland who have been there for at least two weeks.
Authorities in England will carry out spot checks and those breaching the rules could be punished with a fine or prosecution.
Home Secretary Priti Patel told sceptical lawmakers in parliament last week that the measure was "backed by the science, supported by the public, and essential to save lives".
The government is pushing ahead with a gradual lockdown easing that will see retail reopen on June 15, and restaurants and bars begin limited service in early July.
But the devastated hospitality sector relies heavily on tourists, and business leaders fear the quarantine will mean much of the summer season will be lost.
It comes after heavily-hit Italy reopened its borders last week and other European states follow suit.
British Prime Minister Boris Johnson's government is trying to arrange "travel corridors" with countries such as France and Spain that could see them lift their quarantine demands.
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