Speaking at an unscheduled press conference from Downing Street, Dr Vallance said the strain, identified as particularly prevalent in England, has become “the dominant variant” due to the speed with which it can spread.
The strain, which scientists believe mutated from the original virus in mid-September, accounted for around 28 per cent of cases in London and the Southeast of England in November.
But, Dr Vallance explained, by December it had “grown rapidly”, and now represents more than 60 per cent of cases in London.
“So what this tells us is that this new variant not only moves fast, it is increased in terms of its ability to transmit, but it is becoming the dominant variant, it is beating all the others,” he told reporters.
It is thought that the new strain spreads up to 70 per cent faster that the original virus.
The new strain of coronavirus contains 23 separate changes from the original strain identified at the start of the pandemic, which Dr Vallance described as “an unusually large number”.
“It's also got variants in areas of the virus that are known to be associated with how the virus binds to cells and enters cells.”
Dr Vallance added that, as far as scientists can tell, "there's no evidence it causes a more severe disease, causes more hospitalisation, or causes more trouble than the other virus.”
It comes as the Prime Minister announced that from Sunday areas in the South East currently in Tier 3 will be moved into a new Tier 4 – effectively returning to the lockdown rules of November.
Non-essential shops, gyms, cinemas, hairdressers and bowling alleys will be forced to close for two weeks – while people will be restricted to meeting one other person from another household in an outdoor public space.
The rest of England will also see the Christmas “bubble” policy – allowing up to three household to meet up over the holiday period – severely curtailed, applying on Christmas Day only.
Mr Johnson told a Downing Street news conference that he was making the changes with a “very heavy heart”.
He said: “I know how much emotion people invest in this time of year, and how important it is, for instance, for grandparents to see their grandchildren, for families to be together.
“So I know how disappointing this will be. But we have said throughout this pandemic that we must and we will be guided by the science.
“When the science changes, we must change our response.”