Grant Shapps, the transport secretary, said he was taking the “urgent decision” – which also suspends arrivals from Portugal “given its strong travel links with Brazil”.
It comes as the government refused to rule out bringing in tougher social-distancing measures next week, amid concerns that the current lockdown may not be enough to bring the coronavirus outbreak under control.
Asked whether restrictions could be beefed-up, the home secretary, Priti Patel, only said there would be no new measures “today or tomorrow”, fuelling speculations that they would come after the weekend.
Ministers are reportedly considering mandatory mask-wearing outside and a ban on exercising with anyone outside a person’s own household. Increasing social-distancing measures – requiring people to stay three-metres rather than two-metres apart at all time – is also thought to be one of the policies under consideration.
But the latest daily figures raised hopes that the current measures might be enough, as the prevalence of Covid-19 fell in most regions of England and across all age groups apart from the over-80s.
The Public Health England data shows that although London continues to have the highest rate of any region, 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.
Scientists say the mutations of the variant shared with the South African strain appear to be triggering a rapid increase in cases in locations where infections were already very high.
Now arrivals will be banned – from 4am on Friday – from a total of 15 countries, Argentina, Bolivia, Cape Verde, Chile, Columbia, Ecuador, French Guiana, Guyana, Panama, Paraguay, Peru, Suriname, Uruguay, Venezuela, as well as Brazil itself.
However, British and Irish nationals, and third country nationals with residence rights, will be able to fly home – with the requirement that they self-isolate for 10 days, along with their households.
Mr Shapps also tweeted: “Travel from Portugal to the UK will also be suspended given its strong travel links with Brazil – acting as another way to reduce the risk of importing infections.
“However, there is an exemption for hauliers travelling from Portugal (only), to allow transport of essential goods.”
The move comes after Boris Johnson was left red-faced over the Brazil variant during questioning by a committee of MPs – promising he was “taking steps” without saying what they were.
Overnight, there was further embarrassment when new rules requiring travellers arriving in England to have a negative coronavirus test were suddenly delayed to allow “time to prepare”.
The requirement for passengers by boat, train or plane – including UK nationals – to test negative for Covid-19 up to 72 hours before leaving their country of departure was also due to come into force at 4am on Friday.
But it has been pushed back until the same time on Monday, amid concern that guidance on which tests would be accepted had not been published early enough.
Yvette Cooper, chairperson of the Commons Home Affairs Committee, described the delay in introducing the new rules as “truly shocking”.
A lot is still unknown about the new variants, including whether the current vaccines available in the UK will be effective against them.
Even if not, a new coronavirus jab could be manufactured within just 30 to 40 days, according to Nadhim Zahawi, the vaccine development minister.
There have been many mutations in Sars-CoV-2, the virus that causes Covid-19, since it emerged in 2019, because of pressure on the virus to evolve when so many millions of people have now been infected.
Sometimes can lead to weaker versions of a virus – but the new UK variant is more transmissible, just as the Brazilian strain is thought to be.