LONDON — Millions of people must cancel their Christmas get-togethers and most shops have to close in London and much of southern England, British Prime Minister Boris Johnson said Saturday as he imposed a new, stricter level of coronavirus restrictions on the region to curb rapidly spreading infections.

Johnson said Saturday that the capital and large areas in southern England already under the highest level of the U.K. government’s three-tiered coronavirus alert system will move into a new Tier 4 that requires all nonessential shops, hairdressers and indoor leisure venues to close after the end of business hours Saturday.

With just days to go until Christmas, Johnson also announced that a planned easing of socializing rules that would have allowed up to three households to meet in“Christmasbubbles” from WednesdaytoDec. 27willbe canceledforTier4areasand sharply curtailed in the rest of England.

“It is with a very heavy heart that I must tell you we cannot proceed with Christmas as planned,” Johnson said.

He said he concluded there was “no alternative open to me” and people must sacrifice this Christmas to have a better chance of protecting the lives of loved ones.

No mixing of households will be allowed in Tier 4 except under very limited conditions outside in public places. Travel in and out of Tier4areaswon’tbeallowed unless essential. In the rest of England, people will be allowed to meet in Christmasbubblesforjustoneday.

The changes upend the plans of millions of people who were looking forward to gathering with family and friends this week and force scores to revise their travel Shoppers walk past stores Saturday in London ahead of British Prime Minister Boris Johnson’s new lockdown restrictions. TOLGAAKMEN/GETTY-AFP plans at the last minute.

While restaurants, pubs, bars and theaters in much of England are already closed and prepared for a bleak Christmas, all shops in Tier 4 areas that don’t sell food or medicine received only hours’ notice that they must shutter after Saturday until officials review the situation on Dec. 30.

TheBritishRetailConsortium said it was “hugely regrettablenews,” especially coming just two weeks after a monthlong lockdown in England ended and at the height of the year’s peak retail period.

“Retailers have invested hundreds of millions of pounds making stores COVID-secure for customers and staff,” said Helen Dickinson, the trade body’s chief executive. “For businesses, the government’s stop-startapproachisdeeply unhelpful.”

In announcing the more restrictivecategory, Johnson said that a fast-moving new variant of the coronavirus that is more than 70% more transmissible than existing strains appears to be driving the rapid spread in London and southern England.

While London fared relatively well in controlling the virus throughout the fall, the city now has the highest infection rates in England.

Officials said the new mutationaccountedforsome60% of the capital’s cases.

“There’s no evidence to suggest it is more lethal or causes more severe illness,” the prime minister stressed, or that vaccines will be less effective against it.

Viruses mutate regularly, and scientists have found thousandsofdifferentmutations among samples of the viruscausingCOVID-19. But many of these changes have no effect on how easily the virus spreads or how severe symptoms are.

In Wales, authorities said they decided to move up a lockdown planned for after Christmas and people must stay at home from 12:01 a.m. Sunday. The move will largelyscrapChristmasgatherings in line with the rules for southern England.

The Scottish government said Saturday that residents would be subject to Tier 4 restrictions starting Saturday, andatravelbanbetween Scotland and the rest of the U.K.willbeineffectthroughout the holiday period.

Northern Ireland already saidafreshlockdownwould start after Christmas.