U.K. Lockdown, Citing Fast-Spreading Version of Virus

The variant is up to 70 percent more transmissible than earlier versions, officials said. People in southeast England, including London, were told to stay at home.

Alarmed by a fast-spreading variant of the coronavirus, Prime Minister Boris Johnson abruptly reversed course on Saturday and imposed a wholesale lockdown on London and most of England’s southeast, banning Christmas-season gatherings beyond individual households.

The decision, which Mr. Johnson announced after an emergency meeting of his cabinet, came after the government got new evidence of a variant first detected several weeks ago in southeast England, which the prime minister asserted was as much as 70 percent more transmissible than previous versions.

The new measures, which take effect on Sunday, are designed, in effect, to cut off the capital and its surrounding counties from the rest of England. They are the most severe measures the British government has taken since it imposed a lockdown on the country back in March, and reflect a fear that the new variant could supercharge the transmission of the virus as winter takes hold.

“When the virus changes its method of attack, we must change our method of defense,” a somber Mr. Johnson said on Saturday. “We have to act on information as we have it, because this is now spreading very fast.”