British Prime Minister Boris Johnson on Saturday announced a raft of “temporary and precautionary” new rules to combat the spread of the “monstrous” new Omicron variant of the coronavirus.

Speaking in a somber and abruptly called press conference, Johnson and his top medical aides ordered all new UK arrivals to self-isolate until they receive a negative PCR test on British soil.

The rule comes on top of a ban on all flights from a growing list of countries in southern Africa. On Friday, the UK called a halt on travel from South Africa, Botswana, Lesotho, Eswatini, Zimbabwe and Namibia. Four more nations will be added to the red list this weekend, Health Secretary Sajid Javid said Saturday: Malawi, Mozambique, Zambia and Angola.

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.