British Prime Minister Boris Johnson on Saturday announced a "stay at home" order for London and southeast England to slow a new coronavirus strain that is significantly more infectious.
Early data suggests the new strain could be "up to 70 percent more transmissible," Johnson said at a televised briefing.
He ordered new restrictions for London and south-eastern England from Sunday, saying that "residents in those areas must stay at home" at least until December 30.
The measures will mean around a third of England's population cannot travel or meet other households for Christmas.
The aim of the new rules is to slow the spread of the new strain to areas where it is not yet prevalent.
Johnson and his scientific advisers warned the public that the virus can now spread much faster than previously and that existing measures were not enough to control it.
It was a dramatic step-up after Johnson said last week that it would be "inhuman" to "cancel Christmas" by banning family gatherings, though he urged people to have small celebrations.
"It is with a very heavy heart I must tell you we cannot continue with Christmas as planned," Johnson told the nation.
"Alas when the facts change, you have to change your approach."
The announcement came as Britain has seen rising cases and hospital admissions this month.
"This virus has taken off, it's moving fast, and it's leading inevitably to a sharp increase in hospital admissions," said Patrick Vallance, the government's chief scientific adviser.
On Saturday there were 27,052 new cases in the UK, slightly less than on Friday.
Wales' First Minister Mark Drakeford also announced new restrictions from Sunday, with festive "bubbles" - limited gatherings of selected contacts -- allowed only for Christmas Day.
In Scotland, First Minister Nicola Sturgeon announced a ban on travel to other parts of the UK for Christmas and said families in Scotland should mix only on Christmas Day.
Currently people in Northern Ireland are set enter a new lockdown after Christmas.
Under the new lockdown measures that take effect from Sunday morning, those living in the most severely affected areas will be advised not to go to areas with lower restrictions under the country's tiered system, and not to stay overnight. They can still travel for work, however.
Non-essential shops, gyms, cinemas, bowling alleys, casinos, hairdressers and nail bars will be closed in London and other affected regions, including Kent, Buckinghamshire and Berkshire.
Only one person from one household will be allowed to meet one person from another household in a public place, outside.
London Mayor Sadiq Khan said that the capital "faces its toughest Christmas since the war," referring to World War II.
He urged the government to increase testing, give more compensation for lost income and make mask wearing compulsory outside in busy public spaces.
The business community criticised the measure, with Helen Dickinson, the chief executive of the British Retail Consortium, warning that "many thousands of jobs could be at risk."
Vallance said that the new coronavirus strain "contains 23 different changes", including variants that are associated with how the virus binds to cells and enters cells.
By the week beginning December 9, the new strain was accounting for 60 percent of cases in London, he said.
"We think it may be in other countries as well," he said.
England's chief medical officer Chris Whitty said earlier Saturday that the UK had informed the World Health Organization of its findings about the new coronavirus strain.
Downing Street said that it had also informed the leaders of the other regions of the UK.
There was no evidence that the new strain causes more severe illness or higher mortality, Johnson said.
"Equally, there's no evidence to suggest the vaccine will be any less effective against the new variant," he said.