The new strain of coronavirus circulating in Wales is significantly more infectious that the original strain of the virus that spread across the UK, scientists believe.
Scientists have studied the variant - called VUI-2020/12/01 - and say it is 70% more transmissible than the first strain of coronavirus that spread around the UK. That itself is thought to have been different, and more infectious, than the original strain that started in China.
Scientists are concerned that the virus appears to have spread widely in London and the south-east even when lockdown restrictions were in place - meaning it looks likely to be much harder to contain.
The work to study the transmissibility of the new strain was done by a team of scientists form Public Health England, Imperial College in London and Edinburgh University over the last two weeks.
Prime Minister Boris Johnson told a press conference on Saturday, December 19, that the data was still only from early studies and may be reviewed by that the early indications were: "It may be up to 70% more transmissible than the old variant."
John Edmunds, a member of the government advisory body Sage, told ITV's Robert Peston: " This is the worst moment of the whole epidemic, as far as I am concerned.
"This strain has been increasing exponentially during the lockdown and the Tier 3 restrictions that Kent and other parts of the south east have been under...
"We will need much more severe measures to bring the incidence down. Worse than that, we are starting from a very high incidence already with hospitals stretched and NHS staff under strain. It is a very perilous situation."
Asked whether this is the worst moment of the pandemic, England's chief medical officer Chris Whitty told the press conference: "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 [because] although this virus is more transmissible... we do have medical countermeasures.
"If you think back to the dark days at the end of March and April... we had no vaccines on the horizon."
UK chief scientific officer Sir Patrick Vallance said the new variant was first seen in mid-September in London and Kent. He says by December it had become the "dominant variant" in London.
It is understood at least 10 cases have been identified in Wales and many more are expected to be identified.
Sir Patrick said: "Every time a virus replicates, it can make a mistake and so some of these things just arise by chance. The question is what advantage it gives the virus and this one seems to have given the advantage of transmissibility.
"We think it may be in other countries as well. We think there's a large outbreak in the UK, it may have started here, we don't know for sure."
In a statement, Prof Whitty said: "As announced on Monday, the UK has identified a new variant of Covid-19 through Public Health England’s genomic surveillance.
"As a result of the rapid spread of the new variant, preliminary modelling data and rapidly rising incidence rates in the South East, the New and Emerging Respiratory Virus Threats Advisory Group (NERVTAG) now consider that the new strain can spread more quickly.
"We have alerted the World Health Organisation and are continuing to analyse the available data to improve our understanding.
"There is no current evidence to suggest the new strain causes a higher mortality rate or that it affects vaccines and treatments although urgent work is underway to confirm this.
"Given this latest development it is now more vital than ever that the public continue to take action in their area to reduce transmission."