For those already in Britain, all close contacts of people who test positive for the Omicron variant must self-isolate for 10 days, Johnson said — whether or not they have received COVID jabs and boosters — and face masks will be made mandatory again on public transportation and in stores, but not in restaurants.

“We don’t know how effective our vaccines will be” against the fast-spreading new variant that emerged in recent days in southern Africa, Johnson said.

“But we have good reasons to believe they will provide at least some measure of protection,” he said.