A ban on travellers to the UK from South America and Portugal will come into force on Friday due to a new Covid variant identified in Brazil.
Transport Secretary Grant Shapps said the change would come into force from 04:00 GMT.
British and Irish citizens and foreign nationals with residence rights will still be able to travel but must isolate for 10 days, he said.
The decision was announced following a meeting of ministers on Thursday.
The ban applies to people who have travelled from, or through, Argentina, Brazil, Bolivia, Cape Verde, Chile, Colombia, Ecuador, French Guiana, Guyana, Panama, Paraguay, Peru, Suriname, Uruguay and Venezuela in the last 10 days.
Mr Shapps said Portugal had been included “given its strong travel links with Brazil” – but there would be an exemption for hauliers travelling from the country to allow the transportation of essential goods.
He said suspending travel from that European country would act “as another way to reduce the risk of importing infections”.
On Wednesday, Prime Minister Boris Johnson said he was “very concerned” about the Brazilian variant of the virus.
New variants of Covid-19 have previously been identified in the UK and South Africa , with many countries imposing restrictions on arrivals from both nations.
Several Central and South American nations had already restricted travel from the UK, including Brazil, Chile, El Salvador, Guatemala and Panama, while others such as Argentina and Uruguay have closed their borders to non-resident foreign nationals.
The Department for Transport said under the ban any exemptions would not apply, including those for employment, and people who live with someone who has travelled from one of the banned countries will have to self-isolate for 10 days.
BBC News online health editor Michelle Roberts said there was no sign that any of the new variants were more dangerous , with scientists confident that vaccines would work against them.
Felipe Naveca, deputy director of research at the Brazilian state-run Oswaldo Cruz Foundation, said the new variant was “of concern” and its origin was “undoubtedly” from the Amazon region.
He told the BBC’s South America correspondent Katy Watson the new variant had evolved separately from those in the UK and South Africa, but that it showed some of the same mutations.
“Some of these mutations have been linked to increased transmission and that is of concern,” he said, adding: “We have to stop the virus from circulating, because we’re giving it the opportunity to evolve.”
The travel ban comes after it was announced a requirement for arrivals into England to test negative for coronavirus 72 hours before their journey will now come into force at 04:00 GMT on Monday.
Scotland is taking the same approach to international travellers but will implement the policy on Friday, while Wales and Northern Ireland are expected to announce their own plans in the coming days.
Mr Shapps said the new rules had been delayed from Friday “to give international arrivals time to prepare” but Labour leader Sir Keir Starmer said people would be “bewildered” by the delay and would “feel that we’re exposed”.
Meanwhile, Covid-19 case rates have fallen in most regions of England, according to the latest data from Public Health England’s weekly surveillance report.
London continues to have the highest rate of any region but its rate of new cases stood at 864.9 per 100,000 people in the seven days to 10 January, down from 1,043.9 in the previous week.
The North East, East Midlands, East, South East and Yorkshire & the Humber also recorded a week-on-week fall.
However, the rate increased in the North West, South West and the West Midlands.