The new strain of coronavirus currently circulating in parts of England is "out of control," UK Health Secretary Matt Hancock told Sky News on Sunday after Prime Minister Boris Johnson announced stricter restrictions ahead of Christmas.
Hancock added the government acted "very quickly and decisively".
The UK prime minister said on Saturday that London and parts of southeast and east England will move to "Tier 4" restrictions with all non-essential shops closing from Sunday.
People in those areas must stay at home and cannot travel during the holidays and the government is recommending that people in all Tiers should stay local and not travel during Christmas.
"It is with a very heavy heart I must tell you we cannot continue with Christmas as planned," Johnson said at a press conference on Saturday, stating that they "bitterly regret" the changes that need to be made.
The government had originally planned to relax rules allowing three households to meet over five days for Christmas, but that will now be limited to just Christmas Day.
It comes after a really "dramatic increase" in infections in the southeast and London where in some areas, most of the new COVID-19 cases are the new variant, said Professor Chris Whitty, England's chief medical officer.
Whitty said that in other parts of England, this new variant was less present but that due to its transmissibility they were encouraging people not to travel.
"The new variant contains 23 different changes," said Sir Patrick Vallance, the UK's chief scientific officer, who described them as a "constellation of changes". Officials announced that the new strain was much more contagious due to these changes and up to 70% more transmissible.
"We think it may be in other countries as well," added Vallance.
Wales First Minister Mark Drakeford said his nation would also lockdown from Sunday following a meeting with UK officials over the new variant.
In Wales, people from two households can gather in a "bubble", Drakeford said.
"The situation is incredibly serious. I cannot overstate this," he said.
Scottish First Minister Nicola Sturgeon said that some cases of the new variant were also in Scotland and the household mixing would be limited to Christmas Day.
"I know how harsh this sounds," Sturgeon said, stating that the virus was unfair and that the new restrictions made her want to cry. "Please stay home and stay safe this Christmas."
Scotland will apply level four measures from December 26 for three weeks, closing non-essential shops and restaurants.
"There is no current evidence to suggest the new strain causes a higher mortality rate or that it affects vaccines and treatments although urgent work is underway to confirm this," said Whitty in an earlier statement.
"Given this latest development it is now more vital than ever that the public continue to take action in their area to reduce transmission."
The UK prime minister had been criticised for the limits placed on Christmas gatherings with many health authorities stating that they did not go far enough.
The government said last week that they would keep the guidelines for Christmas, relaxing rules to allow three households to come together, but encouraging families to spend the holiday at home.
Days later, they are now tightening restrictions. "Keep it small, keep it short, keep it local and remember the vulnerable," Whitty emphasised.