The new variant of coronavirus could be responsible for up to 60% of infections in Wales, according to the deputy chief medical officer.
Speaking alongside First Minister Mark Drakeford at the Welsh Government's coronavirus briefing, Professor Chris Jones said the new variant could be the "significant driver of the huge growth in cases we’ve seen in Wales in recent weeks."
The new strain is thought to be present across all of Wales and Professor Jones said the mutant Covid strain is 70% more transferable than the original.
He said: "A new analysis by the weekend, suggested this new strain is much more common and it is present all over Wales – this includes North Wales where overall rates of the virus are lower than in South Wales.
"ONS infection survey data published at the weekend shows the new strain was present in 28% of samples from Wales in the second week of December, more than double the number in the previous week.
"Public Health Wales colleagues advise us that they feel this new strain could be causing up to 60% of coronavirus infections in Wales."
First Minister Mark Drakeford said the new variant of Covid-19 provides "better understanding" for the large increase in cases and people being admitted to hospital in Wales from late November.
At the end of November, the seven-day coronavirus rate for Wales was 232 cases per 100,000 people but this is now 623 cases per 100,000 and rising, he said.
There were just under 1,700 people in Welsh hospitals with coronavirus symptoms a month ago but there are now more than 2,300.
"Our critical care units are operating beyond their normal capacity and our NHS and frontline staff are certainly stretched to the limit," Mr Drakeford said.
Over the weekend, more than 100 deaths were reported by Public Health Wales.
"This pandemic has been full of surprises and it’s taken another unpleasant turn, just as we were preparing to celebrate Christmas," Mr Drakeford said.
He continued: " This new strain is another challenge for us, but it’s one we will manage together. Together, we have come through so much this year.
"From those very early days at the start of the pandemic when we just learning about this virus to long summer days, when coronavirus was in retreat.
"The last few months have been challenging, but throughout them all our greatest strength has been you and the way you have responded to the virus to keep each other safe."
The first minister said he would be meeting with trade unions to discuss options for healthcare workers who have to work on Christmas Day to be "compensated."
He told the press briefing: "There will be people who will be working on this Christmas Day...we are discussing with the TUC and others whether it would be possible to offer those people working on that day any compensatory arrangements."
There have been a further 2,563 cases of coronavirus in Wales, taking the total number of confirmed cases to 125,329.
Public Health Wales reported another 10 deaths, taking the total in Wales since the start of the pandemic to 3,125
Mr Drakeford announced on Saturday that an immediate lockdown in Wales was needed to protect people from the new strain of coronavirus.
Wales was due to move into its Level four of restrictions, the highest level available, from December 28, with non-essential retail, close contact services, gyms and leisure centres and hospitality due to close over Christmas Eve and Christmas Day, but Mr Drakeford said it would do so at end of trading on Saturday.
Plans to allow five days of relaxed restrictions allowing up to two families to form a festive bubble have also been cut short and will now only apply for Christmas Day.
As part of the lockdown, the Welsh Government has also issued guidance on what supermarkets can and can't sell - with items such as toys, clothes, electrical goods and gardening products not able to be sold.
What is the new Covid variant?
Viruses mutate over time. In order for them to continue to survive, they adapt and evolve.
Since Covid-19 appeared in Wales, there have been many mutations identified and tracked - it’s an entirely natural thing to happen.
What makes this new variant significant, is that it has mutated in such a way that it makes it better at continuing to spread.
Essentially, this new variant has developed a more effective way for the virus to identify its host’s cells and for its host cells to identify it. It has also developed a better way of passing on its genetic code, and pass it into human cells.
This, in a nutshell, is what makes this new variant better as a virus.
Scientists say that while they’re still researching this new variant, there is no indication at this stage that it causes a more severe illness than what we’ve come to understand about COVID-19 already.
What this means for all of us is that the measures of social distancing, hand washing and mask wearing becomes even more critical.