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, APA reports citing BBC.
Covid passes will also be needed to get into nightclubs and large venues from next week.
Boris Johnson announced the government was moving to its back-up plan of extra Covid rules at a news conference.
"It's not a lockdown, it's Plan B," the prime minister said.
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".
He said the new variant was "growing much faster" than Delta and early analysis suggested cases could be doubling every 2.5 to three days.
The NHS Covid Pass crashed hours after the PM's announcement, with users unable to download their domestic or travel passes. NHS Digital said it was investigating the issue as a priority.
Mr Johnson said Christmas parties and nativity plays should still go ahead - as long as the guidance is followed.
"The best way to ensure we all have a Christmas as close to normal as possible is to get on with Plan B," he said. "Irritating though it may be, it is not a lockdown."
Ministers have repeatedly said there are no plans for another lockdown in England.