People arriving in the UK will now have to self-isolate for 14 days under new rules introduced to stop the spread of COVID-19.
Only people working in specific cross-border jobs, such as lorry drivers, will be exempt from the measure.
It means international holidays may not be possible for the vast majority of British people this summer.
Here is how the system will work.
When does it start?
The measure comes into force on Monday 8 June and will be reviewed by the government every three weeks. The first review will be on 28 June.
Factors that will be assessed include rate of infection and transmission internationally, measures implemented by other countries, and levels of imported cases in other countries with more relaxed border measures.
The quarantine will apply to anyone arriving into the UK by plane, train or ferry.
Why is the quarantine being introduced now?
The number of coronavirus cases in the UK is now falling, but many countries are in earlier stages of their outbreaks.
Places such as the US and Brazil are still reporting thousands of new COVID-19 cases every day.
The government therefore wants to limit the amount of contact international travellers, including UK nationals, have with other people for the first two weeks they are in the UK.
Requiring people to self-isolate for 14 days should ensure that anyone entering the country with coronavirus does not pass it on.
Outlining the government's plans to MPs, Home Secretary Priti Patel said: "The transmission rate in the UK continues to decline and international travel is likely to resume from its record low. Therefore the scientific advice is that imported cases of the virus pose a more significant threat to our national effort.
"Travellers from overseas could become a higher proportion of the overall number of infections in the UK and increase the spread of the disease.
"The government is acting now by taking a proportionate and time-limited approach to protect the health of the British people."
How will it work?
After arriving in the UK, travellers will be asked to go straight to the place where they will self-isolate for 14 days.
The only reasons they will be allowed to leave quarantine is for urgent medical attention, support from social services, food and medicine if they cannot be delivered, a funeral of a close relative or an emergency.
Friends and family can collect travellers from the airport, port or station.
Travellers should only use public transport if they have no other option.
If they do choose to use public transport, they should wear a mask or a face covering over the nose and mouth, and stay two metres apart from others.
Visitors, including friends and family, are not allowed, unless they are providing essential care.
How is it going to be enforced?
Travellers will be asked to fill in a form on arrival, which will include their contact information and an address where they will have to remain for two weeks.
If the traveller does not have somewhere to stay, accommodation will be arranged by the government.
In England, health officials will perform spot checks to ensure compliance with the measures and fines of up to £1,000 will be given, with the threat of prosecution also.
"Passengers require a receipt, either printed or on their phone, to prove they have completed the form," Ms Patel said.
"Border Force will undertake spot checks at the border and may refuse entry to non-resident nationals who refuse to comply. They will have the power to impose a £100 fixed penalty notice to those who don't comply."
The home secretary said Scotland, Wales and Northern Ireland will be setting out their own enforcement approaches.
Who is exempt?
The common travel area with Ireland, the Channel Islands and the Isle of Man will be unaffected by the measure.
The government has also made exceptions for a number of key jobs, including road hauliers, medical officials, Eurotunnel train drivers and aviation inspectors.
Seasonal agricultural workers are among those who are exempt and can self-isolate on the property they are working.
Are we allowed to travel internationally?
The government currently only recommends travelling internationally if it is essential.
Social distancing measures must be followed during your journey and a face covering can be worn as a precaution, especially when indoors, in a crowded area.
The Foreign Office also warns that travellers must be aware that countries may restrict travel and close their borders without notice if the outbreak worsens.
Could 'air bridges' allow some international travel to resume?
Transport Secretary Grant Shapps has backed calls for "air bridges" to be created, and the prime minister is said to be personally in favour of the idea.
This would see agreements sought between countries with similar low numbers of coronavirus cases to let passengers travel without going into quarantine.
Greece and Portugal have both suggested air bridges could be created with the UK to allow tourists to visit over the summer months.
But no such agreements have so far been finalised.
Ms Patel acknowledged the quarantine will cause issues for the tourism industry, but said the policy would be kept under regular review to ensure it remained "proportionate and necessary".
And she said ministers were considering measures that would allow greater freedom in future - including establishing "international travel corridors" with countries that were deemed safe.
What are other countries doing?
New Zealand, South Africa, South Korea, Spain and the US are among countries that already require new arrivals to undergo a 14-day quarantine.
This is despite the World Health Organisation saying in February that measures which "significantly interfere" with international travel "may only be justified at the beginning of an outbreak".
However, some European nations have indicated they would like to start welcoming visitors again soon.
Spain in particular has said it would like to reopen to tourists from late June.
How has the travel industry reacted?
The move has angered many travel and hotel businesses keen to return to some sense of normality, having seen a dramatic fall in customers and bookings during the pandemic.
Airlines are also heavily critical of the measure, which they argue will inflict further damage to an industry already devastated by the coronavirus crisis.
With planes around the world grounded since late March, carriers had hoped to start flying from July, but bosses say
the quarantine rules will hamper that recovery.
Three airlines have written to the government in protest at the "wholly unjustified and disproportionate" step.
Describing itself as a "pre-action protocol letter," meaning it could be followed by legal action, the correspondence from British Airways, Ryanair and easyJet argued the government had failed to justify the blanket nature of the regulations.
"The effect is to establish a wholly unjustified and disproportionate restriction on individuals travelling to
England (and questionably the United Kingdom) and will inevitably mean that there is very little increase in the
numbers of persons leaving and entering the country," the letter said.
It came after Willie Walsh, the chief executive of BA's parent company IAG, told Sky News that the quarantine policy was "terrible" and had "torpedoed our opportunity to get flying in July".
Separately, the chief executive of Wizz Air Jozsef Varadi told Sky News that demand for flights to and from the UK could be hit by up to 80%.