Arrivals into the UK will be expected to quarantine for two weeks from today, in a move that has caused anxiety in the hospitality sector.
Travellers arriving by plane, ferry or train - including UK nationals - must give an address where they will self-isolate.
Rule breakers will be fined £1,000.
Home secretary, Priti Patel, said the laws were designed “to prevent a second wave” of coronavirus.
However, the move has been roundly condemned by the travel sector.
Low-cost carrier Ryanair has branded the process “useless”.
Anyone arriving from the Republic of Ireland, the Channel Islands or the Isle of Man does not have to complete a form or enter quarantine.
There are also exemptions for workers in some industries such as road haulage and medical professionals who are providing essential care.
Rory Boland, editor of Which? Travel, said: “Over the past few weeks, the government has caused endless confusion among travellers over whether holidays can go ahead.
“Even today, as it ushers in 14-day quarantine for UK arrivals, many consumers are confused as to whether the holiday they already have booked will take place due to the lack of consistent communication from the government.
“Meanwhile, the absence of a definitive date from the Foreign & Commonwealth Office on when its travel ban will remain in force until continues to allow travel firms to sell holidays departing in the next few weeks that almost certainly can’t go ahead.
“Not only will those customers not get a holiday, but they may not get their money back either – as some travel firms continue to delay and deny refunds.”
Visitors should drive their own car to their destination, where possible, and once at their destination they must not use public transport or taxis, the government said.
They must not go to work, school, or public areas, or have visitors - except for essential support.
British Airways, easyJet and Ryanair have written to procurator general, Jonathan Jones - the first stage required when taking legal action against the government.
The airlines confirmed they are prepared to ask for a judicial review into the quarantine rules.
In a joint statement the carriers added: “These measures are disproportionate and unfair on British citizens as well as international visitors arriving in the UK.
“We urge the UK government to remove this ineffective visitor quarantine which will have a devastating effect on UK’s tourism industry and will destroy (even more) thousands of jobs in this unprecedented crisis.”
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