People in England are being asked to work from home again if possible and face masks will be compulsory in most public places, as part of new rules to limit the spread of Omicron.
Vaccine passports will also be needed to get into nightclubs and large venues from next week.
Boris Johnson announced the government was moving to Plan B - its back-up plan of extra Covid rules.
“It’s not a lockdown, it’s Plan B,” the prime minister told a news conference.
He said moving to the tougher measures was the “proportionate and responsible” thing to do.
Mr Johnson said more is still being learned about new variant Omicron and the picture might get better, but that it “could lead to a big rise in hospitalisations and therefore sadly in deaths”.
Sir Patrick Vallance - the government’s chief scientific adviser - urged people to follow the rules.
“Measures work when people follow them and that’s been shown time and time again during this,” he said. “It’s incredibly important everyone follows it, that’s everybody. It only works if we all do it.”
Anyone with a connection to No 10, Downing St., is, of course, exempt …