The first guests will check into England's quarantine hotels today as part of the latest measure designed to prevent new cases of COVID-19.
Those arriving in England from 33 high-risk countries will have to quarantine in selected hotels for 10 days, at a cost of £1,750 for a single adult.
Health Secretary Matt Hancock said: "As this deadly virus evolves, so must our defences. We have already taken tough action to limit the spread, protect people and save lives.
"With the emergence of new variants, we must go further. The rules coming into force today will bolster the quarantine system and provide another layer of security against new variants at the border."
So far, 16 hotels have signed up to take part, providing 4,963 rooms, with a further 58,000 rooms on standby, according to the Department of Health and Social Care.
Watch: What are the new UK travel rules?
On Saturday Heathrow Airport had warned "significant gaps" remained in the plan and a spokeswoman said the airport was yet to receive the "necessary reassurances" from the government.
On Sunday night she said: "We have been working hard with the government to support the successful implementation of the managed quarantine policy from Monday.
"Good progress has been made to address a number of issues. Queues at the border in recent days have been almost five hours and this is totally unacceptable.
"Border Force today has given us assurance that they will have 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."
Those required to quarantine can only arrive at Heathrow Airport, Gatwick Airport, London City Airport, Birmingham Airport, Farnborough Airport or any military airfield or port.
And they face fines of up to £10,000 for failing to quarantine, or up to 10 years in jail if they lie on their passenger locator forms.
Watch: Travellers from 'red list' countries will have to pay £1,750 for 10-day hotel quarantine
The quarantine packages must be pre-booked and arrivals must stay in their rooms with exceptions only for urgent medical assistance, exercise, or attending the funeral of a close family member.
These exceptional reasons should only happen with "prior permission by a person authorised by the secretary of state for this purpose".
People coming to the UK from countries not on the hotel quarantine list must isolate at home for 10 days and complete a COVID-19 test on the second and eighth day after arrival.
Critics have questioned whether the hotel quarantine system should be expanded or all inward travel banned, as some countries not on the hotel quarantine list had confirmed cases of the South African variant of COVID-19.
Foreign Secretary Dominic Raab, however, told Sky's Sophy Ridge on Sunday programme: "I'm not sure that's proportionate, and of course having blanket bans on any, for example, air travel into the UK would be very difficult for the supply chains, things like freight."
He added: "We think we've got the right balance - robust measures, but targeted measures."