The mutated strain of coronavirus is spreading fastest in the East of England, government scientists have revealed.
In a government briefing, chief medical officer Professor Chris Whitty said the strain was detected in 1.7% of all positive Covid-19 tests - the highest rate in the country outside London.
He also said around one in 50 people in the UK are estimated to have the virus.
In the briefing, Prof Whitty issued a stark warning about the infection rate and warned restrictions may need to be enforced next winter.
He said: "If we did not do all the things all of us must now do, if people don't take the stay at home seriously, the risk at this point in time, in the middle of winter with this new variant, is extraordinarily high."
Prof Whitty said the risk level will gradually decrease over time with measures being "lifted by degrees, possibly at different rates in different parts of the country".
He added: "We'll then get over time to a point where people say this level of risk is something society is prepared to tolerate and lift right down to almost no restrictions at all.
"We might have to bring in a few in next winter for example, that's possible, because winter will benefit the virus."
Leading the briefing, Prime Minister Boris Johnson confirmed more than 1million people have now received a coronavirus vaccination.
Mr Johnson said more than 650,000 people over the age of 80 - one of the groups most at-risk of serious illness from Covid-19 - had received a jab, offering them a "significant degree of immunity".
The government is also to begin providing updates on how many doses have administered daily from next week, with more information set to be announced on Thursday.
Tuesday's briefing came after the whole of England was placed in lockdown until at least mid-February in a bid to curb the spread of the virus.
Mr Johnson said there is a "prospect" of restrictions being lifted at that time, but said this was dependent on the success of the vaccine rollout programme.
He said: "When a very considerable proportion of the most vulnerable groups have been vaccinated, then there really is the prospect of beginning the relaxation of some of these measures.
"But you will also appreciate there are a lot of caveats, a lot of ifs built into that, the most important of which is that we all now follow the guidance."
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