Wales will introduce new virus measures on Christmas Eve, with high-level restrictions coming into effect from 28 December, First Minister Mark Drakeford has announced.
The Welsh Labour leader said a "sustained rise in coronavirus" meant that the country would have to move into its highest level of restrictions.
Mr Drakeford also said the planned five days of relaxed measures would remain in place following talks between the four UK nations.
Some measures will come into force on Christmas Eve with all non-essential retail, including close contact services and all leisure and fitness centres forced to close from the end of the day.
All hospitality premises will be forced to close from 6pm on Christmas Day.
But tighter restrictions for household mixing, staying-at-home, holiday accommodation and travel will only apply from December 28, with the planned easing of Covid restrictions over Christmas remaining in place between December 23 and 27.
Mr Drakeford told the Welsh government press briefing today: "Many of you will have seen the warnings from senior clinicians about the huge impact coronavirus is having on the whole of our health service.
"The situation we are facing is extremely serious. We must move to alert level four and tighten the restrictions to control the spread of coronavirus and save lives.
"This new set of higher-level restrictions will apply to the whole of Wales.
"This means all non-essential retail, including close contact services and all leisure and fitness centres will close at the end of trading on Christmas Eve.
"All hospitality premises will close from 6pm on Christmas Day.
"And on December 28, at the end of the five day Christmas period, tighter restrictions for household mixing, staying-at-home, holiday accommodation and travel will apply."
Earlier, a senior minister of the UK government said it is to press ahead with the easing of coronavirus restrictions over Christmas even though it will lead to an increase in the infection rate.
Communities Secretary Robert Jenrick said that it would be up to people to make a "personal judgment" whether they wanted to meet up with vulnerable family members over the holiday period.
He suggested that some people may decide to "keep it small" and put off larger gatherings until the spring, saying: "Easter can be the new Christmas."
Nearly 140,000 people in the UK have been vaccinated against Covid-19 in the first week of roll-out of the shot developed by Pfizer and BioNTech, the minister in charge of deployment of the vaccine said this morning.
A really good start to the vaccination program. It's been 7 days and we have done: England:108,000 Wales: 7,897 Northern Ireland: 4,000. Scotland:18,000 U.K Total 137,897. That number will increase as we have operationalised hundreds of PCN (primary care networks)— Nadhim Zahawi (@nadhimzahawi) December 16, 2020
Meanwhile, British Prime Minister Boris Johnson is resisting growing pressure to scrap the easing of restrictions over Christmas amid warnings the move could overwhelm the NHS and contribute to the loss of "many lives".
Cabinet Office minister Michael Gove is resuming talks over the plans for up to three households to mix between 23 and 27 December with leaders of the devolved administrations.
A UK government source acknowledged that England, Scotland, Wales and Northern Ireland may take differing approaches, but insisted there would be no change in the law for the festive freedoms in England.
Instead it was said that the leaders would continue discussions on strengthening warnings, including advising people to stay local and reconsider whether they should spend Christmas with the elderly and clinically vulnerable.
Talks began yesterday after two leading medical journals warned that a lessening of restrictions would "cost many lives", and the British Medical Association (BMA) echoed Labour leader Keir Starmer in demanding an urgent rethink.
They will resume as nearly 10.8 million more people begin living under the toughest restrictions when London and parts of Essex and Hertfordshire moved into Tier 3.
With 61% of England's population now living under the strictest measures, ministers were due to formally review what tiers are appropriate for each area.
In Northern Ireland yesterday, queues of ambulances formed outside several hospitals as pressure continued to mount on the over-capacity health service.
Scotland's First Minister Nicola Sturgeon earlier argued there is a case for reducing the planned freedoms to combat a rise in infections and indicated she could break with the UK approach.
She told the Scottish Parliament: "I do think there is a case for us looking at whether we tighten the flexibilities that were given any further, both in terms of duration and numbers of people meeting."
In Wales, First Minister Mark Drakeford told the Senedd "the choice is a grim one" but said the current plans were a "hard-won agreement" that he would not put aside "lightly".
The meeting was held as the government said a further 506 people had died within 28 days of testing positive for Covid-19, bringing the UK total to 64,908.
Another 18,450 infections were also confirmed in labs as of 9am yesterday.
Mr Starmer had urged Mr Johnson to call an emergency meeting of the government's top-level Cobra committee within 24 hours to assess the situation.
In a letter to Mr Johnson, the Labour leader accused ministers of having "lost control of infections" and warned that "the situation has clearly taken a turn for the worse since the decision about Christmas was taken".
"If you conclude with government scientists that we need to take tougher action to keep people safe over Christmas, then you will have my support," Mr Starmer said.
Earlier, the British Medical Journal and Health Service Journal published a rare joint editorial calling for the "rash" decision to relax social distancing measures over the festive period to be scrapped.
They said the Mr Johnson "is about to blunder into another major error that will cost many lives".
"The government was too slow to introduce restrictions in the spring and again in the autumn," the joint editorial said.
"It should now reverse its rash decision to allow household mixing and instead extend the tiers over the five-day Christmas period in order to bring numbers down in the advance of a likely third wave."
The BMA backed the warning, saying the combination of a third wave in the new year and the typical winter pressures are a "recipe for catastrophe".