On Saturday, the Prime Minister said the government had to take urgent action after it was confirmed that a new coronavirus strain circulating in south-east England, could be up to 70 percent more transmissible than the original.
Johnson has imposed an effective lockdown on more than 16 million people in England and reversed plans to ease restrictions over Christmas.
"It is with a very heavy heart I must tell you we cannot continue with Christmas as planned," he said at a news conference.
"I sincerely believe there is no alternative open to me."
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London and southeast England, which is home to about a third of the English population, are currently in the highest level of a three-tier system of rules. Under the new restrictions, they will now be placed in a new Tier 4 level, similar to those of a recent national lockdown.
From midnight on Saturday, people will be required to stay at home except for essential reasons such as work, and non-essential retail will close, as will indoor leisure and entertainment.
Social mixing will also be limited to meeting one other person in an outdoor space.
Johnson, whose initial response to the pandemic has been criticised for being too slow, had resisted calls to change plans for the Christmas relaxation, saying on Wednesday it would be "frankly inhuman" to ban it.
However, those now in Tier 4 will not be allowed to mix with others over Christmas. And everyone else in England will now be allowed to see friends and family only on Christmas Day itself.
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