A mutated strain of COVID-19 that is spreading more rapidly in the UK has forced the cancellation of Christmas for millions of people in the worst-affected areas.

Prime Minister Boris Johnson said it was with a “heavy heart” that new tier four restrictions – similar to a lockdown – would come into immediate effect in London and south-east England.

After earlier in the week promising to relax restrictions so people could have a “merry little Christmas”, Mr Johnson said Christmas Day would have to be sacrificed.

“When the virus changes its method of attack, we must change our method of defence,” Mr Johnson said.

“This is now spreading very fast,” he warned. “It is with a very heavy heart that I say we cannot continue with Christmas as planned.”

Mr Johnson said the surge in cases was being driven by the mutated variant which “appears to spread more easily and may be up to 70% more transmissible than the earlier strain”.

The variant is responsible for about 60 per cent of cases in London where infections have doubled in a week.

However experts said the changed virus did not appear worse than the original strain nor did it seem to impact the effectiveness of the vaccine which is being rolled out to about two million people a week.

Christmas shoppers in Burlington Arcade in London. Photo: Getty

.From Sunday (local time), areas in the southeast where the new COVID variant has taken hold will be moved from Tier 3 into a new Tier 4 which bans any household mixing over Christmas.

In other areas under Tier 3, mixing will now be restricted to Christmas Day.

“We are sacrificing the chance to see our loved ones this Christmas so we have a better chance of protecting their lives so that we can see them at future Christmases,” Mr Johnson said.

Level four restrictions will also be brought forward to apply across Wales after the country’s health professionals noticed a rapid pattern of transmission “remarkably consistent” with that in southeast England.

“The situation is incredibly serious. I cannot overstate this,” Wales’s First Minister Mark Drakeford said.

The move comes after scientists on the Government’s New and Emerging Respiratory Virus Threats Advisory Group (NervTag) concluded that a mutant strain of the infection 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.

Chief Medical Officer Professor Chris Whitty said people under Tier 4 should not travel to prevent the new variant taking hold in other parts of the country.

Under the new “stay at home” order – covering around a third of the population of England and all of Wales – people in Tier 4 should not leave home overnight and people from outside will be advised not to visit Tier 4 areas.

“Keep it small, keep it short, keep it local, and remember the vulnerable are vulnerable,” Prof Whitty said.

-with AAP