All British and Irish citizens and UK residents who arrive in England after being in a high-risk Covid country now have to self-isolate in hotels.
The “red list” of 33 countries includes Portugal, Brazil and South Africa.
The new regulations, which aim to stop Covid variants entering the country, apply to arrivals who have been in one of those places in the past 10 days.
They have to pre-book and pay £1,750 to spend 10 days in government-sanctioned hotels. The cost covers the hotel stay, transport and testing.
In Scotland, the rule to stay in a hotel applies to travellers arriving directly by air from all countries outside the Common Travel Area (the UK, Ireland, the Isle of Man and the Channel Islands) – rather than just those from the list of 33 countries.
One traveller newly quarantining at the Radisson Blu Edwardian hotel near Heathrow Airport said he was “feeling sad” at the prospect of isolating for 10 days.
Roger Goncalves, 23, from Belo Horizonte in Brazil, said: “I did my test for coronavirus. The test was negative. Why do I need to stay in my room for 10 days?”
Mr Goncalves, a delivery driver who lives in London, flew into the capital from Sao Paolo, via Madrid in Spain because he needed to come back to work.
He said the £1,750 cost of his stay was “too high” and “crazy for 10 days”, but described his room as “not bad” and said he had been told food will be left at his door.
The government has defended the arrangements after questions were raised about the ability of passengers from “red list” countries to mingle with other travellers on flights and at airports before being moved to their accommodation.
A spokesman said: “Separation cannot always be easily implemented at airports, but every step is taken to reduce risks and to minimise any potential for passenger interaction, including Covid-19 tests prior to departure and routine protocols like mandatory mask wearing, social distances and regular cleaning of facilities.
“A number of airports, including Britain’s busiest airport, Heathrow, have introduced additional measures to segregate passengers from red list destinations ahead of the immigration hall to limit the possibility of mixing.”
Earlier, Health Secretary Matt Hancock told Times Radio the hotel quarantine system had been operating “smoothly” since it came into force at 04:00 GMT on Monday.
Meanwhile, Heathrow said in a statement that the measures had come into effect “successfully”, and it would continue to monitor whether Border Force had adequate resource and processes “to avoid unacceptable waiting times and compromising the safety of passengers”.