The UK has now vaccinated more than a million people against coronavirus, but a fast-spreading strain of the disease is now "taking off" around the country, England's chief medical officer has said.
Chris Whitty said the new strain is "rising in all parts of the country" and hospital admissions are "going up very rapidly", as are the number of daily deaths.
Watch ITV News Science Editor Tom Clarke and ITV News Health Editor Emily Morgan with more on the vaccine roll-out:
The new variant is contributing to a huge rise in Covid-19 figures, with more than 1 million people in England now infected with coronavirus and an estimated 1 in 50 people in England having been infected with Covid-19 between December 27 and January 2, according to the Office for National Statistics.
"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 told a Downing Street press conference.
But Prime Minister Boris Johnson provided a glimmer of hope by revealing 1.3 million people in the UK have now received a coronavirus vaccination.
The PM said the figures include 650,000 people over the age of 80, which was 23% of all the over 80s in England.
Watch our info-graphic about the vaccine priority groups:
"That means nearly one-in-four of the most vulnerable groups will have, in two to three weeks, a significant degree of immunity," he said.
"That is why I believe the Joint Committee on Vaccination and Immunisation was right to draw up a programme saving the most lives the fastest."
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He said by the middle of February, if everything goes well, there "really is the prospect of beginning the relaxation of some of these measures".
But he added: "I wouldn't put it any stronger than that."
Mr Johnson said the UK "has already, I think, certainly done more [vaccinations] than any other European country".
But Prof Whitty admitted the UK's new policy of extending the gap between vaccination doses could cause further coronavirus mutations.
To be inoculated effectively, people must receive two doses of the vaccine. After the first dose people are thought to have around 50% more protection from the virus.
Rather than fully vaccinating people by giving them two doses within three weeks of each other, the UK has decided to extend the gap between jabs in order to give more people their first dose, and a further level of protection.
Prof Whitty said: "By extending the gap we are going to, over the next three months, be able to essentially double the number of people who are vaccinated.
"If over that period there is more than 50% protection, then you've actually won. More people overall will have overall been protected than would have been otherwise."
But he added there is a "theoretical risk that by having this longer gap, you could actually lead to a slightly increased risk of an escape mutant".
"All vaccination will ultimately put pressure on viruses to mutate", he said, but the risk is "small".
On Monday evening the PM announced England was being put into a strict, six week lockdown, the rules of which are similar to restrictions under the first lockdown.
It means people in England will only be allowed to leave their home for essential reasons, until February 15 at the earliest.
It is hoped the vaccination programme will help the UK ease out of lockdown, but two new variants of the virus are causing serious concern among experts and ministers, with it not yet clear whether vaccines will have an effective impact on them.
The UK variant, which is thought to spread around 70% more quickly than the original, is fuelling a new surge of infections across the UK.
The other variant, which is thought to have originated in South Africa, is believed to be even more fast spreading, however it is currently much less prevalent in the UK.
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Sir Patrick Vallance said it is possible the South African coronavirus variant may have some impact on vaccine effectiveness but is unlikely to "abolish" their effect.
The chief scientific adviser told the Downing Street press conference that a possible change in the virus shape in the variant "theoretically gives it a bit more risk of not being recognised" by the immune system.
"There is nothing yet to suggest that's the case. This is being looked at very actively," he said.
He added: "At the moment, you'd say the most likely thing is that this wouldn't abolish vaccine effect. It may have some overall effect on efficacy but we don't know."