The emergence of the Brazil Covid variant in the UK could lead to the end of lockdown being delayed or even reversed, a top scientist has suggested.
SAGE advisor Graham Medley said there was always a risk "we might have to go backwards" when variants of concern emerge here.
It comes after a second expert warned there were concerns the P.1 variant might "evade" antibodies and make the vaccine "less potent".
Boris Johnson has announced a four-stage plan for lifting lockdown, in which he has insisted he does not want to go backwards. But he has also made clear the dates in his roadmap are not fixed - and will be delayed if needs be.
Pub beer gardens could have to open later than April 12 and overnight stays later than May 17 if key tests - which include the emergence of new variants - are not met.
But the PM said he was 'confident' the roadmap wouldn't be slowed down by the Brazil cases.
And he 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."
It comes after a desperate race was launched to find a mystery Brit with the P.1 strain - who may not have known they were positive or stayed in isolation.
The case is one of six that were detected on or around Friday from tests done in mid-February - three in England, and three in Scotland. The other five either flew into the UK from Brazil, or live with someone who did.
But the sixth person did not fill in a test card so the test has no name attached. It raises the threat that the variant could already be spreading in the community and genome sequencing takes about 10 days.
The people arrived days before the long-delayed hotel quarantine finally took force for Brits returning from Brazil.
P.1 has been deemed a ‘variant of concern’ because it shares important mutations with the South Africa variant - including E484K and N501Y.
Scientists believe those mutations may make it respond less well to the vaccine, though there is no data on this yet.
It is also possible it may spread more easily than other variants of coronavirus, like the Kent variant which has become dominant in the UK.
Prof Medley, professor of infectious disease modelling at the London School of Hygiene and Tropical Medicine, said challenges, such as the new Brazilian variant, could mean the nation needs to "go backwards" in terms of relaxing restrictions.
He told BBC Radio 4's Today programme: "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.
"So monitoring these variants, keeping an eye on in terms of what they actually do - so sequencing, for example, viruses in hospitals - I think is a crucial step to know whether or not this variant and other variants in the future, what impact they're actually having."
Imperial College London's professor of immunology Danny Altmann said vaccines may be less effective against the variant.
"It's somewhat more worrying than the UK variant, the Kent variant, that we're used to talking about, because it covers the double whammy, we think, of being more transmissible and somewhat better at evading neutralising antibodies,” he warned.
"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."
Dr Susan Hopkins of Public Health England added: "Manaus in particular reported that a number of individuals were re-infected with this variant.
"Therefore that suggests that having had prior immunity from primary infection wasn't enough to reduce infection and transmission.
"And that may also impact on the vaccine."
But Prof Medley added the variation of Covid-19 in different parts of the country could be a bigger challenge for the Government than the new variants.
Graham Medley, a member of the Scientific Advisory Group for Emergencies (SAGE), told the BBC: "In a way that's going to be the bigger challenge for the Government going forward than the variants.
"I think that we are already seeing, and when we start opening up we will see more, variation in terms of prevalence around different parts of the country.
"At the moment all the thinking that I've seen - but I'm sure there's others - that has been largely national in terms of thinking about what the data are that we need to guide the process of releasing these measures.
"But the data will show different things in different parts of the country, and so the challenge will be what do you do in terms of opening things up when in one place it says it's a good idea, in other places it says it isn't?
"These are very old problems, these social inequalities and health inequalities have existed for a very long time."
He added: "We'll have to look at those regional variations because they are likely to be just as big as they were last summer and in the autumn."
