The first travellers required to stay at quarantine hotels have begun arriving in the UK.
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.
Hotel cost 'too high'
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.
Meanwhile, Zari Tadayon, 66, from north London, faces spending her birthday in isolation after flying in from Dubai, via Frankfurt.
Asked how she felt about spending 10 days in isolation, she said: "I feel horrible because I live here, I have my own individual home, and also I have some medical issues which I was hoping they would consider.
"I'm not prepared. I didn't bring books and stuff."
She added that she wasn't feeling happy "because tomorrow is my birthday and I would have wanted to be with my family... those are the rules, what can you do?"
In Scotland, the first passenger to go into quarantine hotel was Chun Wong, an American medical worker, who had travelled from America to Edinburgh via Dublin with his eight-year-old daughter Kiernan - to join his wife in Fife.
He told the BBC he would do "whatever it takes" to stay, adding: "Even though I got my Covid shot already, whatever it takes to make everybody safe."
However, he and his daughter were later allowed to leave the hotel after it was realised that as they had arrived from the US - which is not on the red list - via Dublin, which is in the Common Travel Area, they were allowed to isolate at home instead.
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".
It comes after the airport warned of delays caused by the rules.
Are you travelling to the UK from a "red list" location? Tell us by emailing email@example.com.
Heathrow Airport is one of five in England where people requiring hotel quarantine can enter the UK and is expected to receive the largest number of passengers. The other four are Gatwick, London City, Birmingham and Farnborough.
Passengers arriving at London's Heathrow Airport on Monday were escorted by security personnel to coaches, which took them to nearby hotels, including the Radisson Blu Edwardian hotel.
At Birmingham Airport, there were just four flights touching down on Monday, including a plane from Istanbul with about 100 passengers, some of whom are believed to be from a red list country.
The government says it has struck deals with 16 hotels so far, providing 4,963 rooms for the new quarantine system, with a further 58,000 rooms currently on standby.
People travelling from red list countries to Wales and Northern Ireland will be required to book and pay for quarantine in England, as neither destination has any direct international flights.
Any passenger required to stay in a quarantine hotel in England needs to reserve a room online in advance using a government portal.
The additional rate for one extra adult or a child aged over 12 is £650, and for a child aged five to 12 it is £325.
Those who fail to quarantine in such hotels face fines of £5,000 to £10,000, while anyone who lies on their passenger locator form about having been in a country on the red list faces a prison sentence of up to 10 years.
Asked at a No 10 briefing on Monday what would happen if someone staying in a hotel had to extend their stay due to a positive test, Prime Minister Boris Johnson stressed it was currently illegal to travel abroad for holidays and that people going into quarantine should be able to "cover their costs".
A new testing regime for all travellers arriving in England has also started, with two tests required during the quarantine process.
They will be required to get a test on days two and eight of their 10-day quarantine period, whether they are isolating at home or in a hotel. The tests, conducted by NHS Test and Trace, will cost travellers £210.
Those who do not take the tests could face a £2,000 fine.
Scotland's First Minister Nicola Sturgeon said the nation had decided to adopt stricter rules "because we think it is important to go as far as we can".
At her daily Covid briefing, she also stressed a common approach to travel restrictions across the four nations of the UK would be "preferable" and she said Scottish ministers would try to persuade the UK government "to adopt more comprehensive measures".
Welsh First Minister Mark Drakeford said he also wanted to see "a stronger set of defences at our borders" and described the UK government's measures as "the bare minimum of what needs to be done".
Last week, Labour's shadow health secretary said the public wanted the government to "go further" on border quarantine measures.
Travellers arriving in the UK - whether by boat, train or plane - are already required to show proof of a negative Covid-19 test to be allowed entry.
And all travellers - including British nationals - must self-isolate for 10 days when they get to the UK.
Lockdown rules mean people must only travel abroad for essential reasons.
The 33 red list countries are:
- Cape Verde
- Democratic Republic of Congo
- French Guiana
- Portugal (including Madeira and the Azores)
- South Africa
- United Arab Emirates (UAE)