People in all parts of the United Kingdom will, from Tuesday, have to wear face masks while inside shops and using public transport, after measures aimed at slowing the spread of a new strain of the novel coronavirus were announced.

Because of the strain, which the World Health Organization named Omicron after it was identified in southern Africa late last week, the UK government said people arriving at its borders will have to take a PCR test and isolate until they can produce a negative result.

In addition, people who come into contact with someone suspected of having the Omicron strain will be told to self-isolate for 10 days, regardless of their vaccine status.

The UK's prime minister, Boris Johnson, said during a news briefing on Saturday that the caution was the result of preliminary indications that vaccines work less well on the new strain, but he said more work must be done to establish just how virulent and dangerous it is.

"Our scientists are learning more, hour by hour, and it does appear that Omicron spreads very rapidly, and can be spread between people who are double-vaccinated," he said.

The Guardian newspaper noted that Omicron had already spread to the UK, where there were two cases as of Sunday morning, and to the European nations of Belgium, the Czech Republic, Germany, and Italy.

Johnson said the UK's new measures will only delay its spread, not keep it out.

"We need to slow down the spread of this variant here in the UK, because measures at the border can only ever minimize and delay the arrival of a new variant, rather than stop it," he said.

The UK's health secretary, Sajid Javid, added on Sunday: "The reason we've set out these measures… is to protect the progress we've made, so we can all continue to enjoy Christmas with our families."

He explained on The Andrew Marr Show that the government decided to stop short of forcing people to work from home and carry vaccine passports, but said they could be added in three weeks, when the government conducts a review.

The additional measures, which are known as Plan B, will be introduced if the National Health Service comes under extreme pressure, he said.

"We know now that those types of measures do carry a very heavy price, both economically, and socially, in terms of non-COVID health outcomes and the impact on mental health," he said.

The BBC said Javid wants to hold off considering Plan B until scientists have established how serious the threat is.

The UK's main opposition, the Labour Party, said the government should, instead, plan for a worst-case scenario and implement Plan B.

The Financial Times newspaper said Javid is also waiting to hear from vaccine advisers on the effectiveness of current jabs, and about whether the nation's booster program should be extended to cover more people.

The BBC said Usdaw, the shopworkers' union, complained the government seemed to be putting the economy ahead of people's health.

Paddy Lillis, a spokesman for the union, said: "Retail staff working with the public every day are deeply worried about catching COVID-19 and the arrival of the Omicron variant is a further concern."

The Financial Times newspaper noted that the UK government's chief medical adviser, Chris Whitty, believes there could be "vaccine escape" with the new strain, which means it may be able to spread, regardless of the vaccination program.

The FT said it will likely take two to three weeks for results to come through from tests on the effectiveness of vaccines on the new strain.