Anyone arriving in the UK will be asked to take a PCR test on the second day and must self-isolate until they provide a negative test in a bid to combat the Omicron Covid variant.
Prime Minister Boris Johnson announced the move at a press conference on Saturday, after two confirmed cases of the "worst ever strain" were discovered in Essex and Nottingham.
Mr Johnson also revealed that anyone with a suspected case of Omicron will have to isolate for 10 days - regardless of their vaccination status.
He said: "We're not going to stop people travelling, I want to stress that, we're not going to stop people travelling, but we will require anyone who enters the UK to take a PCR test by the end of the second day after their arrival and to self-isolate until they have a negative result.
"Second, we need to slow down the spread of this variant here in the UK, because measures at the border can only ever minimise and delay the arrival of a new variant rather than stop it all together.
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"We will require all contacts of those who test positive with a suspected case of Omicron to self-isolate for 10 days regardless of your vaccination status.
"We will also go further in asking all of you to help contain the spread of this variant by tightening up the rules on face coverings in shops and on public transport."
Mr Johnson said that the measures would be reviewed in three weeks, when scientists should know more about the new strain.
"We need to protect gains we have made so far," he added.
"This is the responsible course of action to slow down the seeding and the spread of this variant."
Health officials revealed earlier on Saturday that two cases of the feared Omicron variant had been found in the UK - one in Brentwood, Essex and another in Nottingham.
The UK Health Security Agency confirmed the cases, which are both believed to be connected and linked to travel to southern Africa, after genomic sequencing overnight.
The individuals and their households were ordered into self-isolation and targeted testing was being carried out in areas where they are thought to have been infectious.
Health Secretary Sajid Javid said "this is a real reminder that this pandemic is far from over" as he urged people to get their vaccines, including boosters.
"We were concerned from the moment we first identified this new variant," Mr Javid told reporters.
"It's a deeply concerning new variant and we do need to learn more about it but the fact that we now have these two cases in the United Kingdom does mean we need to take further measures and that's why I've set this out today."
Mr Javid did not say whether further restrictions could be added in the weeks before Christmas in light of the presence of the new strain, which the World Health Organisation (WHO) has designated a "variant of concern".
Instead, he said: "We've made a lot of progress, we all want to see that protected and if anyone's sitting at home thinking what can I do? Get vaccinated."
Essex County Council said the case within its boundaries was identified in Brentwood, and not Chelmsford as incorrectly first stated by the Department of Health and Social Care, and is linked to the case from Nottingham involving international travel to South Africa.
The government has now added four more southern African nations to the travel red list: Malawi, Mozambique, Zambia and Angola.
Mr Javid said anyone who has travelled in the last 10 days to the 10 countries on the red list, which also includes South Africa, Botswana, Lesotho, Eswatini, Zimbabwe and Namibia, must self-isolate and take PCR tests.
The UK is the second European nation to have reported the presence of Omicron after Belgium said it had identified a single case on Friday.
The Netherlands was also of concern, with Dutch authorities saying 61 people tested positive for Covid-19 arriving on two flights from South Africa on Friday. Further tests are under way to determine if any of them had the Omicron variant.
It was unclear when the two individuals were infected, or their full travel history, as countries scrambled to close their borders to much of southern Africa.
Ministers said non-UK and Irish residents who have been in Malawi, Mozambique, Zambia and Angola in the previous 10 days will be refused entry into England from 4am on Sunday.
Those who are permitted to return will be ordered to isolate in a Government-approved facility for 10 days.
Numerous pharmaceutical firms have said they are working to adapt their vaccines in light of the emergence of Omicron after the WHO warned that preliminary evidence suggests the variant has an increased risk of reinfection and may spread more rapidly than other strains.
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The detection of the cases came after Professor Sir Andrew Pollard, who helped create the Oxford/AstraZeneca vaccine, expressed optimism that existing vaccines will be effective at protecting against serious disease caused by the new Omicron variant.
He also said it is "extremely unlikely" it will cause a "reboot" of the pandemic in Britain as he offered tentative hopes the current jabs will still confer some degree of protection against the variant.
Prof Sir Andrew, who also chairs the Government's Joint Committee on Vaccination and Immunisation, said a new vaccine could be developed "very rapidly" if required because they now have a "well-oiled" process.
He told BBC Radio 4's Today programme that most of the mutations in Omicron are in similar regions seen in other variants, adding: "That tells you that despite those mutations existing in other variants the vaccines have continued to prevent serious disease as we've moved through Alpha, Beta, Gamma and Delta.
"At least from a speculative point of view we have some optimism that the vaccine should still work against a new variant for serious disease but really we need to wait several weeks to have that confirmed.
"It's extremely unlikely that a reboot of a pandemic in a vaccinated population like we saw last year is going to happen."
Professor Calum Semple, who advises the Government as part of the Scientific Advisory Group for Emergencies (Sage), also offered some optimism by saying the current evidence is "it's not causing more death".
"The problem this might present is it might evade some of the vaccines but it might not evade the boosters or the two proper doses," he told BBC Breakfast.
But some experts were more concerned and ministers were facing calls to go further to prevent a wave of the new variant arriving in Britain while a Delta surge is ongoing.
Professor John Edmunds, who also sits on Sage, warned that could create a "very, very, very difficult situation" and said "all the data suggests" it would be able to evade current immunity, telling BBC Radio 4's PM programme: "Our fears are it would do so to a large extent."
Experts at the WHO said there is early evidence to suggest Omicron has an "increased risk of reinfection" and its rapid spread in South Africa suggests it has a "growth advantage".