Six people in the UK have tested positive for a concerning variant of coronavirus which was first found in the city of Manaus in Brazil. Most of them had flown from Brazil before UK border measures were tightened.
Unlike other countries, the UK has never banned international travel altogether, and only brought in tougher measures - including quarantine hotels - relatively recently.
What are the current travel rules?
Only essential travel is permitted under the current lockdown rules, with international holiday travel not allowed until 17 May at the earliest.
From 8 March, passengers who want to travel internationally must fill out and carry a form setting out whether their trip is permitted. They will be fined if they fail to do this.
All UK arrivals must provide a recent negative Covid test and a passenger declaration form with their contact details, and then quarantine for 10 days.
Direct flights from 33 high risk "red list" countries - including South Africa and Brazil - are banned, to help protect the UK against concerning coronavirus variants.
British and Irish citizens who've been in red list countries can still enter the UK via indirect flights, but are taken to a hotel where their 10 days of isolation can be monitored.
In Scotland, hotel quarantine applies to all travellers arriving from outside the Common Travel Area.
Travellers must also have to take two coronavirus tests while quarantining, and will have to self-isolate for longer if they get a positive result.
How were the Brazil cases able to get through?
Coronavirus variants originally found in Brazil and South Africa are believed to be more transmissible, and there are concerns vaccines may not be quite as effective against them.
Although direct flights from these countries are banned, British and Irish citizens who've been in these countries are allowed to return to the UK and until recently were trusted to self-isolate at home - and take public transport to get there.
A monitored hotel quarantine was announced on 27 January, but didn't come into effect until almost three weeks later. It has emerged that by then, six people in the UK had tested positive for the Brazil variant - five linked to travel from Brazil.
Prime Minister Boris Johnson said the UK had moved "as fast as we could" to introduce hotel quarantine measures, describing it as a "very tough regime".
Labour said there had been a lack of a "comprehensive" border system.
The UK checks about 25,000 positive test results each week for variants using genome sequencing. Any positive test from a passenger arriving in the UK is automatically sent for sequencing.
The process takes around ten days on average but there are concerns that by the time a variant is identified it may have spread.
Why didn't the UK shut its borders before it had any cases?
The debate over whether to shut UK borders stretches back to the start of the pandemic.
Between January and March 2020, the UK introduced some measures to try to protect itself from the new virus.
Quarantine was imposed on 273 people travelling from Wuhan in China, where the first outbreak was seen. Others from "high-risk countries" including China, Iran and Northern Italy were asked to voluntarily isolate for 14 days.
An inquiry into the pandemic suggested it was understandable the government "did not consider it practical or effective simply to restrict flights" at that time, partly because there were so many British residents trying to get home.
But these self-isolation requirements were withdrawn on 13 March 2020 - 10 days before the UK went into lockdown.
"The government's failure to have proper quarantine measures in place in March as the infection was spreading fast was a grave error and meant Covid spread faster and reached more people," Yvette Cooper, chair of the committee of MPs behind the inquiry said.
One study found the virus was introduced to the UK "well over a thousand times in early 2020", with a third of transmission chains brought in from Spain, followed by 29% from France, neither of which faced any restrictions. China accounted for just 0.4% of imported cases.
What were the government's reasons?
The Home Office said the committee behind the inquiry was "incorrect in their assertions".
It said keeping borders open helped businesses maintain international connections, boosted the economy and meant people could still travel for essential reasons.
It argued that by mid-March there was "significant transmission" in the UK and any extra travel restrictions at that time would have had a "very marginal" impact.
At a meeting in February 2020, the government advisory committee Sage suggested that cutting the number of infections entering the UK by half would only delay the epidemic by five days.
The scientists estimated that a 95% reduction in imported cases, which would have required "draconian" travel restrictions, would have pushed things back by a month.
What have other countries done?
Australia and New Zealand closed their borders to almost all visitors in March, although travellers from New Zealand can now enter most Australian states without quarantine.
This is currently a one-way agreement - meaning they must do 14 days of managed isolation on their return. And Australians are not allowed in at all, unless they have an exemption such as being in a relationship with a New Zealand resident.
Most foreign nationals are not allowed into Japan unless they are residents. Even then, they have to provide evidence of a recent negative test, get tested on arrival and then isolate for 14 days. Travellers from the UK must spend the the first three days of isolation in a government facility and take another test on the third day.