"Christmas bubbles" of up to three households will only be allowed to gather on December 25, the prime minister has announced.

In a briefing this afternoon, Boris Johnson announced that in light of the emergence of a new strain of coronavirus the five-day reprieve, which allowed up to three households to form a festive bubble, had to change.

Also announcing a fourth tier of restrictions for several areas of the country, the PM announced that the previous plan for Christmas was no longer a safe approach, due to a rise in cases and the new variant of Covid-19.

He said: "We must, I'm afraid, look at Christmas again as prime minister it is my duty to make difficult decisions to do what is right to protect the people of this country.

"Given the early evidence we have on the new variant of the virus it is with a heavy heart to tell you we can not continue with Christmas as planned.

"In England, the Christmas rules allowing up to three households will now be limited to Christmas Day only rather than the five days as previous set out."

He added there will be no reprieve for new year celebrations.

He added: "I know how much emotion people invest in this time of year, and how important it is, for instance, for grandparents to see their grandchildren, for families to be together.

"So I know how disappointing this will be. But we have said throughout this pandemic that we must and we will be guided by the science.

"When the science changes, we must change our response."

Meanwhile, the PM also announced the introduction of a fourth tier with tougher restrictions applying to some areas currently in Tier 3 - including London, the South East and parts of the East.

However, with Norfolk and Suffolk currently in Tier 2, these new restrictions, this does not apply here.

It does, however, prevent people from parts of the country going into Tier 4 prevented from visiting family in other tiers.

The move comes after scientists on the Government's New and Emerging Respiratory Virus Threats Advisory Group (NervTag) concluded that the mutant strain identified by Public Health England - known as VUI2020/01 - was spreading more quickly.

The prime minister was advised of the group's conclusions at a meeting with ministers on Friday evening, and the new regulations were signed off by the Cabinet in a conference call on Saturday lunchtime.

England's chief medical officer Professor Chris Whitty said people considering leaving Tier 4 areas now should unpack their bag and stay at home.

"My short answer would be 'please unpack it at this stage'," he said.

Prof Whitty told a Downing Street news conference: "If they were to go with this new variant, unwittingly, to an area that has a low prevalence and starts this being seeded even more outside the high prevalence areas that would be a significant risk to the area they went to.

"That is the reason we are really keen that people do not go from these areas."

He indicated that Tier 3 and Tier 2 restrictions had proven ineffective at controlling the new variant.

If the new variant spread "none of them would be able to hold things with Tier 3 and Tier 2 elsewhere".

Labour leader Sir Keir Starmer said he was "really frustrated" following the announcement that Christmas would be cancelled for many families across the UK.

"The infection rates are going up very very fast and so something has to be done, and we support the steps the Government has put in place but millions of families are going to be heartbroken by this news - having their Christmas plans ripped up," he said.

"I'm really frustrated because I raised this with the Prime Minister on Wednesday and he dismissed that and went on to tell people to have a merry little Christmas - only three days later to rip up their plans.

"I think the British public is entitled to more decisive leadership than that."