Travellers entering the UK are expected to face queues of up to five hours and will be able to mix with passengers from 'red list' countries, under new quarantine rules that came into effect on Monday.
Heathrow Airport has warned of long queues at Border Control and said there were no plans in place to separate passengers from the 33 high-risk countries from others.
Officials estimate that checks carried out to see if a traveller has arrived from one of the Government's 'red list' zones, including South Africa, Portugal and the UAE, could take up to 15 minutes per arrival.
A Heathrow spokesman said: “Our key concern remains the ability of Border Force to cope.
“Queues at the border in recent days of almost five hours are totally unacceptable.
“Ministers need to ensure there is adequate resource and effective processes at the border to avoid compromising the safety of passengers and those working at the airport, which could necessitate the suspension of some arriving flights.”
Health minister Matt Hancock said in statement: "As this deadly virus evolves, so must our defences ... The rules coming into force today will bolster the quarantine system and provide another layer of security against new variants at the border."
Mr Hancock said the quarantine system appeared to be working "smoothly" as the first flights arrived into the country.
"As of 6.30, when I got my latest update, this is working smoothly, we've been working with the airports and with the border force to make sure everybody knows the process," Hancock told Times Radio on Monday.
Speaking to Sky News, Mr. Hancock also hailed the UK's vaccine campaign as he confirmed the top four priority groups have been offered a vaccine. He said the milestone was "little step towards freedom for us all".
Letters are being sent to those aged over 65 and the clinically vulnerable so they can book their vaccinations, he said.
He confirmed the UK was looking at vaccine passports which would allow Britons to travel abroad safely in the future.
Meanwhile, Boris Johnson has been urged to reopen international travel by May 1 and support millions of travel jobs in a campaign launched by Save Our Summer (SOS).
The travel alliance group - which includes firms Trailfinders, easyJet Holidays and Celebrity Cruises - says the prime minister must provide a roadmap out of the pandemic to help the struggling travel industry.
It says anyone booking overseas holidays should be entitled to either a refund or a change of date if travel is cancelled over Covid-19 restrictions.
Curently, UK nationals or residents currently in 'red list' countries will have to stay in one of 16 hotels, at a cost to them of £1,750 ($2,422) for 10 days.
The price includes a testing programme while holed up at the hotel.
Britons planning to travel back to the UK have also criticised the high cost of the programme.
Some of the hotels on the scheme have been offering a single-night stay for £65 to visitors not subject to the restrictions.
People must stay in their rooms, except to exercise, get urgent medical care or attend the funeral of a close family member.
The official requirements for hotel operators show the UK system will be less stringent than Australia's in several ways.
Nadine Houghton, of the GMB trade union, said that people getting off planes from the red list countries could be put together with staff and other passengers.
"Our members working at Heathrow Airport, the ground staff, security staff, have been raising concerns about this for two weeks now. Heathrow just isn't safe at the moment," she told The Observer newspaper.
“We’re talking about the spread of new strains, so people entering from different countries into a small space is not a good look.”
One British expat, oil worker David Taylor, told the National the scheme was "farcical" and is considering cancelling a visit to see his wife and children in north-east England next month because of the restrictions.
“I’ve lost 11 nights with my family,” Mr Taylor said.
Other families who hoped to send children from the UAE to the UK in preparation for the potential restart of lessons at private schools have put off the decision because of concerns about the quarantine rules.
A young family returning to the UK after four years in Abu Dhabi complained that travellers were being used as guinea pigs.
Beckie Morris, 30, plans to travel back to the UK with her husband Matthew and their five-week-old daughter but faces a bill for thousands of pounds, the Sun newspaper reported.
Charlie Islam-Harry, manager of the St Giles Heathrow hotel, said staff would do regular welfare checks on guests during their enforced stays.
"It can't be easy for them to be in a bedroom for 10 days, not having to go anywhere," she told AFP.
"But obviously while they are quarantined we try to keep as much distance as possible because we obviously don't know their health situation."
A government spokesperson said: "We are working closely with airports and hotels to manage any issues that arise and ensure the new process runs as smoothly as possible, and we are clear the safety of all staff and passengers is a priority."
