Prime Minister Boris Johnson is holding a coronavirus press conference from Downing Street, with an update expected on Covid-19 rules at Christmas.
At Prime Minister's Questions he announced there had been an agreement "in principle" between all nations of the UK, that the plan allowing three households allowed to mix for five days over Christmas would go ahead.
But minutes later Welsh First Minister Mark Drakeford announced the rules would be tightened in Wales, reducing mixing rules to two households.
He also announced Wales will start moving into stricter level 4 restrictions - the equivalent of a lockdown - from 6pm on Christmas day.
At the press conference, in which the PM will speak alongside the Chief Medical Officer Chris Whitty, Mr Johnson will likely aim to clear up confusion about rules across the UK.
The prime minister's official spokesman said any update in guidance by devolved administrations was a "matter for them".
"We have been clear on what the Christmas rules are but we have said this isn't us telling people what to do, it is up to individuals to ensure that they remain cautious," the spokesman said.
Significant pressure has been mounting for a rethink of the restriction relaxation over Christmas, with two leading medical journals warning a loosening of the rules would “cost many lives”.
On Tuesday evening, Michael Gove held a meeting with the leaders of the UK's devolved administrations to discuss whether the Christmas rules should be tightened.
The emergence of a new, fast-spreading strain having been identified added fuel to calls for Christmas plans to be abandoned.
The new variant could be contributing to significant rises in virus prevalence in London, parts of Essex and parts of Hertfords, which are being placed into Tier 3 of restrictions overnight.