Travellers from South America are no longer allowed to come into the UK, amid fears over a new coronavirus variant first identified in Brazil.
The UK's new travel ban - which also applies to Portugal and Cape Verde - came into force at 04:00 GMT on Friday.
Like variants discovered in the UK and South Africa, it is thought the Brazil variant could be more contagious.
Virologist Prof Wendy Barclay said one Brazilian variant had already been detected in the UK.
However, she said this was not "the variant of concern", which is thought to be more infectious.
Prof Barclay, head of G2P-UK National Virology Consortium, which is studying the effects of emerging coronavirus mutations, said: "There are two different types of Brazilian variants and one of them has been detected and one of them has not."
She added: "The new Brazilian variant of concern, that was picked up in travellers going to Japan, has not been detected in the UK.
"Other variants that may have originated from Brazil have been previously found."
Earlier, Transport Secretary Grant Shapps had told BBC Radio 4's Today programme that the Brazilian variant of concern was not "as far as we are aware" already in the UK, adding that he did not believe there had been any flights from Brazil in the last week.
It comes as a further 1,248 people with coronavirus have died in the UK.
Latest government figures on Thursday also showed another 48,682 new cases had been reported.
Meanwhile, the number of people in the UK to have received the first dose of a vaccine is now approaching three million.
The UK's new travel ban applies to people who have travelled from, or through, Argentina, Brazil, Bolivia, Chile, Colombia, Ecuador, French Guiana, Guyana, Paraguay, Peru, Suriname, Uruguay and Venezuela in the last 10 days.
It also applies to Portugal - because of its strong links to Brazil - and the former Portuguese colony of Cape Verde off the coast of west Africa, as well as Panama in central America.
British and Irish citizens and foreign nationals with residence rights are still allowed to return - but must isolate for 10 days.
Also exempt are hauliers who are travelling from Portugal to transport essential goods.
Brazil has seen more than 200,000 deaths and there is concern about the impact the new mutation could have on its health system.
However, the UK's travel ban was prompted by fears of how quickly the new variant could spread through the region - since Brazil borders 10 countries.
Mr Shapps has said the ban is "precautionary", adding he "can't provide an end date" to the new rules.
"We're so close now, we've got three million of these vaccines in people's arms in the UK," he told BBC Breakfast.
"We want to make sure we don't fall at this last hurdle."
Because holidays are not currently allowed, Mr Shapps said he did not "expect a large number of Brits to have jaunted off to South America", and the government was "not expecting to see a big repatriation issue as a result".
One family, who live in Wolverhampton, told the BBC they feared being stuck out in Brazil.
"I don't know if the government will organise flights," said Jon Dent, 31. He and his wife Carla travelled to the Brazilian city of Goiania in October to introduce their baby daughter to Carla's family.
"I think it's a long shot," he said. "I hope we can get home and not be stranded out here for months. We've got to be patient but at the same time flexible."
Several Central and South American nations - including Brazil - had already restricted travel from the UK before the latest ban on arrivals.
There is currently no evidence to suggest that any of the variants cause more serious illness, and scientists are confident that vaccines should work against them.
According to Felipe Naveca, deputy director of research at the Brazilian state-run Oswaldo Cruz Foundation, the new variant's origin was "undoubtedly" from the Amazon region.
He told the BBC's South America correspondent Katy Watson the new variant showed some of the same mutations as the UK and South Africa variants - and "some of these mutations have been linked to increased transmission and that is of concern".
Mr Shapps also announced Qatar and the Caribbean islands of Aruba, Bonaire, Sint Eustatius and Saba were being removed from the UK's travel corridor list, meaning arrivals from those places will need to self-isolate for 10 days from 04:00 GMT on Saturday.
Meanwhile, France has cracked down on the type of tests that travellers can take to show they are negative.
From Monday, travellers will need to show a negative PCR test. Antigen tests - which are the rapid lateral flow tests - will no longer be accepted.
However, Mr Shapps said arrangements allowing hauliers to use rapid lateral flow tests before crossing the border from the UK into France remained in place at the moment.
From Monday, everyone travelling to England and Scotland will also have to show proof of a negative test. Wales and Northern Ireland are expected to announce their own plans in the coming days.
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