Under fire for imposing an effective lockdown on more than 16 million people just days before Christmas, Health secretary Matt Hancock said Saturday's decision was taken after evidence showed the new strain was responsible for spiralling COVID cases in the UK.
Prime Minister Boris Johnson tore up plans to allow three households to mix indoors for five days over the festive period, imposing new Tier 4 curbs similar to a recent national lockdown on London and the southeast.
Hancock suggested the tougher measures - which require about a third of the population of England to stay at home except for essential reasons such as work - might stay in place until vaccinations become more widely available.
"We've got a long way to go to sort this," Hancock told Sky News.
"Given how much faster this new variant spreads, it's going to be very difficult to keep it under control until we have the vaccine rolled out."
Keir Starmer, leader of Britain's opposition Labour Party, said while he supported the new measures, "yet again the prime minister waited until the 11th hour to take this decision".
"The alarm bells have been ringing for weeks but the prime minister chose to ignore them," Starmer said.
"He told the country to go ahead and have a merry little Christmas ... and yet three days later he tells millions of families to rip up those plans."
The UK's other nations also took action.
Scotland said on Saturday it would impose a ban on travel to the rest of the United Kingdom, with the Christmas easing limited to December 25 only.
All of Wales will go into Tier 4 from midnight, but two households can mix on Christmas Day.
Like other countries in Europe, Britain is battling to contain new waves of the virus. It reported 27,052 new cases on Saturday, taking the total over two million, and 534 more deaths, taking the overall official toll to more than 67,000.
Ministers say the new strain - VUI202012/01 - which has also been identified in Denmark, the Netherlands and Australia, is up to 70 per cent more transmissible than the original but that there is no evidence that it is more lethal or causes more severe illness.
Soon after Johnson told the nation of the changes on Saturday, some in London headed for the capital's train stations to try to travel to see relatives over Christmas, and there were scenes of crowding - something Hancock called "totally irresponsible".