The government's COVID-19 operations committee met at lunchtime to discuss the issue - and the ban came into effect at 4am on Friday.
Transport Secretary Grant Shapps tweeted that arrivals from the following South American countries would be banned: Argentina, Brazil, Bolivia, Cape Verde, Chile, Colombia, Ecuador, French Guiana, Guyana, Panama, Paraguay, Peru, Suriname, Uruguay and Venezuela.
Travel form Portugal is also suspended "given its strong travel links with Brazil", but there is an exemption for hauliers travelling from there to allow the transport of essential goods.
The measure also does not apply to British and Irish nationals and third country nationals with residence rights - but passengers returning from these destinations must self-isolate for 10 days along with their households.
Mr Shapps later announced that Madeira and the Azores would be removed from the travel corridors list from 4am on Friday to try and reduce the spread of the variant.
In addition, people arriving into England from Qatar and the Caribbean islands of Aruba, Bonaire, Sint Eustatius and Saba from 4am on Saturday will have to quarantine for 10 days.
Mr Shapps said the UK had acted because it does not want to "trip up at this late stage" in the pandemic as the vaccine rollout continues.
"We're so close now and we want to make sure we do everything possible to give us the best chance of beating this virus, which is why it's so important that we do act quickly on these things," he said.
The transport secretary added that ministers did not expect there to be a large number of Britons who will now need to return home.
"We don't expect a large number of Brits to have jaunted off to South America and we're not expecting to see a big repatriation issue as a result," Mr Shapps said.
"Certainly people should have been very aware before travelling anywhere. Please don't travel at the moment. You should be staying at home, we're under a lockdown and that applies across the country."
The transport secretary said he could not provide an idea of how long the ban will last for - and did not rule out Spain joining Portugal on the list due to its links with the continent.
"With Spain, there's already restrictions, which includes, for example, quarantine and the necessity from this weekend to make sure that you've taken a test before leaving. We'll keep these things constantly under review."
Travel bans have previously been introduced for those coming from Denmark and South Africa, in response to other new variants that have been detected in those countries.
The discovery of another variant in the UK prompted many countries around the world to introduce travel bans for arrivals from the UK in the run-up to Christmas.
Home Office minister Victoria Atkins earlier told Sky News the government had "acted decisively" in the past following the development of new variants in other parts of the world.
Asked why travel between Brazil and neighbouring countries to the UK had not yet been closed off, she said: "Of course, people flying into the UK, whether from South America or elsewhere are required to have a 10-day quarantine period when they land in the UK. That is mandatory.
"In terms of the decision on travel measures, it takes a little bit of time.
"What we need to ensure is that when we make these very, very important decisions that have a huge impact on people's personal lives, but also businesses, we have got to have a little bit of time to let that bed in.
"The prime minister was clear that measures will be taken, we have acted decisively in the past with both the Denmark and South African variants, so I wouldn't want to speculate further at this stage."
At Christmas, Brazil was among a number of countries to suspend all flights from or via the UK due to the emergence of a new COVID variant in Britain.
Prime Minister Boris Johnson on Wednesday told MPs that the government was "concerned" about the new Brazilian variant.
Mr Johnson said there were "lots of questions" over the Brazil variant, including whether it is resistant to COVID vaccines.
Meanwhile, the government is also under pressure after it delayed the introduction of pre-departure testing requirements for all international travellers to England.
People arriving in England from abroad will now have to prove they have tested negative for coronavirus from Monday and not Friday as originally planned.
Arrivals will still have to quarantine for 10 days regardless of their test results.
Mr Shapps said the delay was to "give international arrivals time to prepare" after the full guidance was only published on Wednesday.
This is despite the Department for Transport having announced the plans for new testing requirements last week.
Labour MP Yvette Cooper, the chair of the House of Commons Home Affairs Committee, posted on Twitter: "No guidance out with 2 days to go. And now delays.
"What on earth are they doing?"