As the world scrambled to prevent the spread of the new Omicron coronavirus variant that was first detected in southern Africa, Britain’s Health Security Agency confirmed on Saturday that two cases of the variant had been recorded in the country.

In a news conference on Saturday evening, Prime Minister Boris Johnson said that to curb the spread of the variant, face masks would be required in stores and on public transportation, a rule the country had ended in July.

“As always, I must stress this, with a new variant there are many things we just cannot know at this early stage,” Mr. Johnson said.

“It does appear that Omicron spreads very rapidly and can be spread between people who are double vaccinated,” he added. Although the science around Omicron is still new, it is a “very extensive mutation” of previous configurations of the virus that could reduce vaccine effectiveness, Mr. Johnson said.

The cases are said to be linked to travel in southern Africa, the British government confirmed in a statement. Sajid Javid, Britain’s health secretary, described the new cases as a “stark reminder” that the pandemic was not yet over.

“Thanks to our world-class genomic sequencing, we have been made aware of two U.K. cases of the Omicron variant,” Mr. Javid said. “We have moved rapidly, and the individuals are self-isolating while contact tracing is ongoing.”

The country’s health agency is now carrying out targeted testing at several locations where infections could have been spread. Britain will also require travelers from abroad to get a PCR test within 48 hours of their arrival and require contacts of those who test positive with a suspected case of Omicron to self-isolate for 10 days, regardless of vaccination status.

“We don’t yet exactly know how effective our vaccines will be against Omicron, but we have good reasons for believing they will provide at least some measure of protection,” he said.

In addition to the six countries in southern Africa that were added to Britain’s travel ban list on Friday to prevent the spread of the Omicron variant, four other countries — Angola, Malawi, Mozambique and Zambia — were also added. Being on the list requires travelers to quarantine in a government-approved hotel for 10 days.