Lockdown restrictions will come into effect in Wales from midnight on Saturday, while festive bubbles will only apply on Christmas Day.
First Minister Mark Drakeford said the rules would mean non-essential retail, close contact services, gyms and leisure centres and hospitality would close at “end of trading” on Saturday.
His announcement came after a meeting with the first ministers of Scotland and Northern Ireland, as well as Michael Gove, on the new strain of coronavirus.
The new strain is “significantly more infectious and spreads more quickly” than the original one, Mr Drakeford said.
He detailed how the pattern of transmission in London and the South East of England, which has been linked to the new variant, is “remarkably consistent with the rapid acceleration of transmission in Wales” and the high case rates seen in recent weeks.
Mr Drakeford said the latest evidence suggested that the new strain is “present throughout Wales”.
“Today has been one of those days when new information has required an immediate response,” Mr Drakeford said.
“This afternoon, the Cabinet met to discuss this worrying new development in the pandemic and to hear the latest advice from our senior medical and scientific advisers, including the impact on our NHS.
“The situation is incredibly serious. I cannot overstate this.
“We have therefore reached the difficult decision to bring forward the alert Level four restrictions for Wales, in line with the action being taken in London and the south east of England.
“These new restrictions will come into effect from midnight tonight instead of during the Christmas period.”
The restrictions mean non-essential retail, close contact services, gyms and leisure centres and hospitality will close at the end of trading on Saturday.
Stay-at-home rules will also come into effect from midnight.
Mr Drakeford said the easing of restrictions over Christmas had to be altered as people could not be exposed “to the risk of this new, more virulent strain of coronavirus”.
“We will therefore change the current rules, which allow two households to come together to form a Christmas bubble over a five-day period, so that they will apply on Christmas Day only,” he said.
“Throughout the alert Level four period, a single person household will be able to join with one other household.
“While we all want to avoid further disruption to businesses and plans for Christmas, our overriding duty is to protect lives here in Wales.
“We know that 2021 will be a different and a better year. Our economy will recover. Christmas will come again. But lives which are lost, are lost for ever.
“This new strain of the virus is another dreadful surprise in this long-running pandemic. We now have a pandemic within a pandemic, a crisis within a crisis.
“It is another challenge we must overcome. But one we will overcome together. We will continue to protect ourselves and our loved ones, and, together, we will keep Wales safe.”
Paul Davies, leader of the Welsh Conservatives, said he had been briefed by Mr Drakeford on Saturday and could “fully appreciate the need for more severe restrictions in Wales”.
“I know that many people will be disappointed as they were looking forward to seeing their loved ones this Christmas, but in the face of the emerging evidence it is clear that Christmas bubble arrangements must change,” Mr Davies said.
“By working together and keeping to the rules we can ensure as many people as possible will be around to celebrate Christmas together in the years to come.”