Urgent talks are taking place between the leaders of the devolved administrations and Cabinet Office Minister Michael Gove as calls mount to scrap plans for an easing of Covid-19 restrictions over Christmas.
However, Wales' First Minister Mark Drakeford described the four nations approach to the festive period as a "hard-won agreement" and said he will "not lightly put it aside".
The move comes as Labour leader Sir Keir Starmer demanded an urgent review into the easing of rules after leading medical journals warned that a lessening of restrictions would "cost many lives".
The planned five-day "Christmas window" would allow three households to mingle between December 23-27.
Mr Drakeford told the Welsh Parliament: "I have a meeting later today with the First Minister of Scotland, the First and Deputy First Minister of Northern Ireland and Michael Gove as the minister in charge of the Cabinet Office, no doubt this issue will be discussed.
"The choice is a grim one, isn't it.
"I think the choice is an incredibly difficult one. At the moment we have a four-nation agreement. I will discuss that later today, we will look at the figures again together."
Labour leader Sir Keir urged Prime Minister Boris Johnson to call an emergency meeting of the Government's top level Cobra committee within 24 hours to assess the situation.
The move came as Downing Street said that advice to the public was being kept "under constant review".
Sir Keir's intervention followed a rare joint editorial by the British Medical Journal and Health Service Journal calling for the "rash" decision to relax social distancing measures over the festive period to be scrapped.
"We believe the government is about to blunder into another major error that will cost many lives. If our political leaders fail to take swift and decisive action, they can no longer claim to be protecting the NHS" @fgodlee@HSJEditorhttps://t.co/cbgphEQVjr
— The BMJ (@bmj_latest) December 15, 2020
They said that the Government "is about to blunder into another major error that will cost many lives".
In a letter to the Prime Minister, the Labour leader said the Government had "lost control of infections", stating: "I understand that people want to spend time with their families after this awful year, but the situation has clearly taken a turn for the worse since the decision about Christmas was taken.
"It serves no-one for politicians to ignore this fact.
"It is my view that you should now convene Cobra in the next 24 hours to review whether the current relaxation is appropriate given the rising number of cases.
"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."
The Labour leader wrote: "This is a critical moment for our country. The tiered system has not kept the virus under control and has left us with precious little headroom. Put simply, if you take the wrong decision now, the ramifications for our NHS and our economy in the new year could be severe."
In response to suggestions that the Christmas arrangements could be restricted to three days or two households, Downing Street again said the measures were being kept under "constant review".
The Prime Minister's official spokesman said: "We have set out the guidance for the Christmas bubbling arrangements.
"But... we obviously keep all advice under constant review."
The joint editorial warning, authored by HSJ editor Alastair McLellan and BMJ editor-in-chief Fiona Godlee, said: "When Government devised the current plans to allow household mixing over Christmas it had assumed the Covid-19 demand on the NHS would be decreasing.
"But it is not, it is rising, and the emergence of a new strain of the virus has introduced further potential jeopardy.
"Members of the public can and should mitigate the impact of the third wave by being as careful as possible over the next few months. But many will see the lifting of restrictions over Christmas as permission to drop their guard.
"The Government was too slow to introduce restrictions in the spring and again in the autumn.
"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 relaxation of regulations looms as the Government is putting London, much of Essex and part of Hertfordshire under the strictest Tier 3 restrictions from Wednesday.
Chief Secretary to the Treasury Stephen Barclay said "all things were kept under review" as he urged people to show restraint at Christmas.
London Mayor Sadiq Khan told Sky News: "I heard the Government say yesterday they haven't ruled out further changes, I would encourage the Government to look at their rules over Christmas.
"What I say to the Government is: I'm not sure you've got it right, in fact, I'm sure you haven't got it right in relation to the relaxations over Christmas."
Mr Barclay said people need to "do the minimum" if they visit family over Christmas.
The minister said the easing of restrictions over Christmas was not a "tier zero" situation.
He said: "The position is not that we are scrapping the tiering system for five days – that we are letting people loose.
"What we are saying is, within the family three households can come together. That's the limit of the flexibility that is being offered.
"I think it's been mischaracterised as, almost, a tier zero, that we are scrapping restrictions in their entirety. That is not the case."
The comments come after scientists warned that the easing of coronavirus restrictions over Christmas will cause a spike in infections.
Health Secretary Matt Hancock has said people should be "extremely careful" about who they mix with in the run-up to Christmas if they are planning to see elderly relatives over the festive period.