A member of the government's Scientific Advisory Group for Emergencies (Sage) said the new strain, which has been designated a "variant of concern", could force the UK to "go backwards" in terms of relaxing restrictions.
Graham Medley, professor of infectious disease modelling at the London School of Hygiene and Tropical Medicine, said: "It is a variant of concern but we are going to be faced with these in the next six months as we move towards relaxing measures.
"There are going to be challenges on the way - and there is always a risk that we might have to go backwards, and that's what nobody wants to do is to actually open up and then have to close down again," he told the BBC.
Prime Minister Boris Johnson sought to reassure the public about the new strain, saying there is no evidence to suggest it is resistant to vaccines.
"We don't have any reason at the present time to think that our vaccines are ineffective against these new variants of all types," he said.
But a number of experts have expressed concern about the variant - which was first detected in the city of Manaus - after a case was identified in the UK but officials were unable to locate it.
Danny Altmann, professor of immunology at Imperial College, when asked how worried people should be about the emergence of the variant, said: "Somewhat worried but not total panic, perhaps."
He told Times Radio it looks likely the variant is "breaking through" antibodies built up by previous infection.
He said it was expected that the Manaus population "had one of the highest levels of immunity in the world" due to high infection levels earlier in the pandemic, "and yet they're seeing this enormous wave of reinfections".
"So, if you put two and two together their assumption is that's because the new variant is breaking through those antibodies. But if that hadn't really been proven yet, it looks likely."
On the impact of the strain on vaccine rollout, he said: "When I look at the data on how well this variant gets neutralised, it's not that all immunity is gone, it's that the vaccines look so much less potent, so there'll be more people who have low antibody responses where it can break through and get affected. It all comes back much harder."
Vaccines minister Nadhim Zahawi, asked if the new strain could impact the planned route away from restrictions, said one of the tests that must be met before measures can be lifted is whether new strains mutate to escape vaccines.
Pressed as to whether that means it could delay the plan, he told ITV News: "One of the reasons we have those four tests - and the prime minister outlined very clearly that we will look at four weeks plus one, ie five week intervals between each step of reopening the economy - is to allow us the time... to look at the behaviour of the virus."
Vaccines minister on whether new variant will delay roadmap out of lockdown:
He added: "We will take it deliberately cautiously and slowly as we open up, but it will be driven by the data."
On Monday it was revealed 20,275,451 people have had a Covid-19 vaccination, with 815,816 of those having had a second dose.
The prime minister said he still expected the road map for easing England's restrictions would be irreversible.
He told reporters at a school in Stoke-on-Trent: "What we are doing is embarking now on a journey, a one-way road map to freedom and it is designedly cautious in order to be irreversible.
"That is what we are hoping to achieve. Some people say we should go faster, some people say we should be more hesitant.
"I think we are going at the right pace, education is the priority, getting all schools open on March 8 is something that we have set our hearts on for a long time and I am confident we will be ready."
Officials are scrambling to try and locate the first people in the UK believed to have contracted the newest variant of coronavirus to arrive in the country, as the government faces criticism over its border controls.
Public Health England (PHE) said on Sunday that six cases of the concerning P.1 variant first detected in the Brazilian city have been confirmed in Britain, three in England and three in Scotland.
Two were confirmed in South Gloucestershire but the third English case has not been located and could be anywhere in the nation, with PHE saying the person did not complete their test registration card so their contact details are absent.
Dr Susan Hopkins, strategic response director at Public Health England, said that although cases have been detected in the UK, it is hoped that it would not become a dominant variant.
Mr Zahawi said community transmission of the variant first identified in Brazil will be identified “very, very quickly” through testing.
ITV News Political Reporter Shehab Khan on the search for the missing case:
“One of the strengths of the UK’s system is obviously our genome sequencing capability; we account for just shy of 50% of all the sequencing of the different variants of Covid-19,” he told BBC Radio 4’s Today programme.
“We would pick up, as we have done obviously in these cases, pick up rapidly, both in terms of PCR testing capability – 800,000 capacity per day – and millions of lateral flow tests.
“We would pick up community transmission of this variant very, very rapidly, because we are able to genome sequence so quickly.”
Anyone who took a test on February 12 or 13 and has not received a result, or has an uncompleted test registration card, is being asked to come forward immediately, as health officials scramble to track down the individual.
Critics, however, have said the development exposes the “weaknesses” in the border protections against new strains - condemning the government for not bringing in restrictions more quickly.
The Gloucestershire cluster was said to originate from one individual who travelled back from Brazil and arrived in London on February 10 – five days before the government’s quarantine hotel policy came into force.
The traveller isolated at home with the rest of their household under the rules in place at the time. One member exhibited Covid symptoms before getting a test.
It is understood there were four positive tests in total in that household, two of which were confirmed with genetic sequencing to be the P.1 variant.
But officials are awaiting the results of sequencing on the outstanding two tests to see if they were infected by the Manaus strain.
PHE and NHS Test and Trace are contacting the passengers on Swiss Air flight LX318 travelling from Sao Paulo, through Zurich, and landing in London Heathrow on February 10.
Surge testing will be carried out in the Bradley Stoke, Patchway and Little Stoke areas of South Gloucestershire.
Surge testing will begin in South Gloucestershire from Monday for residents who live in five postcode areas:
The remaining unlocated case is not believed to be linked to the others because the virus was found to have slight genetic differences.
Officials said the individual’s test was processed on February 14, so believe it is likely they took it a day or two earlier.
They believe that person is unlikely to have taken their test at one of the regional test sites, where staff can check if contact details have been provided, but it could have been a home test or from local surge testing.
The Scottish Government said three residents who returned to north-east Scotland from Brazil, via Paris and London, subsequently tested positive for Covid-19.
The tests, completed in early February, were passed to the UK’s sequencing programme and were identified as being the Manaus variant.
Officials are contacting the other passengers on their flight from London to Aberdeen.
The cases are not thought to be connected to the three confirmed cases in England.
The World Health Organisation has been informed of the cases, which have been designated “of concern” as the strain shares key mutations with the variant detected in South Africa.
More work is needed but there are concerns that the existing vaccines may be less effective against the Manaus strain.
NHS England national medical director Professor Stephen Powis sought to allay concerns by saying vaccines can be quickly altered to tackle new strains.
“The new vaccines which are being used for Covid can be adapted very rapidly so it’s likely that if we do need to change the vaccine that can be done in months, rather than years, which was the case with the more traditional vaccines,” he told BBC News.
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It is understood that officials became aware of the English cases on Friday and the Scottish ones on Saturday.
The chairwoman of the Commons Home Affairs Committee, the Labour MP Yvette Cooper, said: “This troubling development shows the weaknesses in the Government’s Covid border measures.
“The Brazil variant was first identified a month before one of these cases was brought in on February 10 and many weeks after the Prime Minister was warned that indirect flights were a problem, yet the Government delayed putting stronger measures in place.
“We need to know urgently how all these cases have arrived in the country and why they weren’t prevented or picked up on arrival so that lessons can be quickly learnt and policies changed to protect the vaccine programme from further cases arriving.”
The quarantine hotel policy to protect the UK against new variants came into force on February 15 after much delay.
It means those returning from “red list” countries, including Brazil, must quarantine in government-designated hotels for 10 days at a cost of £1,750 for a single adult.
Scottish Health Secretary Jeane Freeman said: “The identification of this new variant is a concern but we are taking every possible precaution.
“This new variant demonstrates how serious Covid is and reinforces the need to minimise the spread of the virus.”