THE new variant of coronavirus sweeping London and the South East is "beating all the others in terms of transmission," the Government's chief scientific adviser has said.

Sir Patrick Vallance warned that the new variant was becoming the dominant strain, with a rapid rise in cases in recent days.

"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," he told the Downing Street press conference.

"So there are some changes which cause concern in terms of how the virus looks."

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Sir Patrick said current evidence showed the variant "has a significant, substantial increase in transmissibility" after Prime Minister Boris Johnson said it was up to 70 per cent more transmissible.

Sir Patrick set out how, in the middle of November, about 28 per cent of the cases in London and the South East - and slightly lower in the East of England - were due to the new variant.

"By the week commencing the ninth of December these figures were much higher," he said.

"So in London, over 60 per cent of all the cases were the new variant.

"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 in terms of transmission.

"So this virus transmits and spreads fast."

However, Sir Patrick said there was "no evidence" the variant caused more severe disease or increased the risk of being admitted to hospital.

The "working assumption at the moment from all of the scientists" is also that vaccines should work against it, he added.

"The big change therefore is not disease progress, not immunity, but transmission - this virus spreads more easily, and therefore more measures are needed to keep it under control."

The Government's chief medical officer, Professor Chris Whitty, told the press conference the tier system was holding cases in other parts of the country but the new variant posed a risk to tiers.

"If you have a low amount of this variant, the rate of increase is held by the tiers, but if you have a very high rate of this variant, then it is not held sufficiently by the tiers and it is going up rapidly," he said, adding that people should not travel and risk spreading the variant around the country.

He said there was a risk of the variant "going out to other areas of the country where it currently is not a problem".

Asked whether the current measures would work, he said it was like "you were cycling up a steep hill and now you've got the wind against you as well - you just have to do that much more to actually keep going".

But he said measures such as social distancing and limiting contact were the ones to use.

Prof Whitty said that while the spread of the new variant was another "terrible" moment in the epidemic it was not the worst, because a vaccine being rolled out.

"Is it the worst moment? Well I'm afraid there have been so many terrible moments in this epidemic, this is another one.

"But I have to say in my own view this is not the worst moment in the epidemic and the reason for that is although this virus is more transmissible and we must do everything we can... we do have medical counter-measures.

"We have a vaccine already being rolled out as the Prime Minister said, and therefore there is a prospect, relatively in the medium-term future, where things could be quite a lot better."

Dr Jeffrey Barrett, director of the Covid Genomics Initiative at the Wellcome Sanger Institute, said: "This new variant is very concerning, and is unlike anything we have seen so far in the pandemic.

"The new restrictions announced today are an entirely justified response to the rapidly developing situation.

"Detecting new variants like this is one of the key missions of the Covid-19 Genomics Consortium in the UK.

"The high level of genomic surveillance we have had to date has helped detect and respond to this variant as quickly as possible. We will continue to monitor this and other variants in the weeks and months to come."