Coronavirus cases are up 25 per cent on last Saturday as 27,052 Britons tested positive - pushing the number of cases in the UK past two million, it has been revealed today.
Latest figures show 534 people died in the UK after testing positive for coronavirus - 14 more than last Saturday and 45 more than yesterday.
The total number of lab-confirmed cases of coronavirus in the UK has now passed more than two million since the outbreak began, according to the Government's dashboard.
Sir Patrick Vallance said the new coronavirus variant may have originated in the UK, but that other types of variants around the world were likely.
'This (variant) is a particular constellation of changes, which we think is important,' he said.
'We think it may be in other countries as well, we think it is based mainly, there is a large outbreak in the UK, it may have started here we don't know for sure.'
He added that global 'surveillance' was needed to sequence the new variations.
England's Chief Medical Officer Chris Whitty said the variant strain had been discovered through Public Health England's genomic surveillance and it was now confirmed that it spread more easily that the original version.
'There's no evidence that it causes more severe illness or higher mortality, but it does appear to be passed on significantly more easily,' Johnson told a news conference to announce tougher lockdown restrictions for millions of people.
'Although there's considerable uncertainty, it may be up to 70% more transmissible than the old variant, the original version of the disease. This is early data and it's subject to review.
'But it's the best that we have at the moment and we have to act on information as we have it, because this is now spreading very fast.'
London and the South East are set to be plunged into Tier 4 as a new 'highly contagious' coronavirus variant rips through the regions. Pictured: Oxford Street today
Cases in Britain have soared in the last two weeks, and were rising fast, data showed.
Vallance said COVID-19 vaccines appeared to be adequate in generating an immune response to the variant of the coronavirus.
He said in parts of England, including London, the south east and east of England, the new variant was becoming the dominant form.
'This virus has taken off,' he said of the mutation. 'It's moving fast and it's leading inevitably to a sharp increase in hospital admissions.'
'There is no evidence it causes a more severe disease, causes more hospitalisation, causes more trouble than the other virus, it basically looks similar.'
News of the latest crackdown came as countless Britons flocked to the country's high streets (Oxford Street, pictured) and supermarkets today as they stocked up on Christmas essentials
Meanwhile, the PM was accused of 'inconsistency and confusion' after the extraordinary last-minute U-turn to put a third of England, including London and swathes of the home counties, into a brutal new 'Tier 4' from midnight.
The draconian bracket means non-essential shops being forced to shut, and travel restrictions including a 'stay at home' order for Christmas Day itself - even though Mr Johnson insisted just days ago that it would be 'inhuman' to axe five-day festive 'bubbles'.
The rest of England will not escape unscathed, with up to three households now only allowed to mix on Christmas Day rather than between December 23 and 27.
Wales has also announced it is going into a full lockdown from midnight and will follow suit by slashing bubbles to a single day. Nicola Sturgeon is set to make a statement shortly.
Boris Johnson is taking a Covid press conference today as he creates a new Tier 4 to tackle a new 'highly contagious' coronavirus variant
But there was a wave of anger from MPs and families that the decision has come just six days before Christmas, when millions have already made plans, booked travel and bought food for reunions.
What are the new Tier 4 rules?
The Tier 4 rules will be essentially the same as the blanket lockdown that England was under in November.
Non-essential retail must close, as well as leisure facilities, and personal care such as hairdressers.
However, places of worship can stay open.
People in other Tiers will be advised not to go into the highest bracket areas, while residents of Tier 4 must not stay overnight in lower infection spots.
A clearly uncomfortable Mr Johnson told the nation tonight that he had 'no alternative' but to act after being presented will compelling evidence only yesterday about the devastating spread of the new strain.
Although it does not appear to be any more deadly, it is believed to be much more contagious, accounting for an incredible 60 per cent of new cases in London over the past week.
Mr Johnson said it could increase the crucial R rate by 0.4 and be 70 per cent more transmissible than previous versions.
'We must act now,' he said, appealing for the public to 'stay local' and 'raise a glass for people who are not there'. 'We cannot continue with Christmas as planned....
'I know how much emotion people invest in this time of year and how important it is for grandparents to see their grandchildren and families to be together.
'So I know how disappointing this will be. But I have said throughout this pandemic that we must and will be guided by the science.'
He added: 'As your Prime Minister, I sincerely believe there is no alternative open to me. Without action, the evidence suggests infections would soar, hospitals would become overwhelmed and many thousands more would lose their lives.'