Britain announces a third case of the Omicron variant as the health secretary defends decision on restrictions.

British health officials said Sunday that a third case of the new Omicron coronavirus variant had been discovered in the country, in an individual who had spent time in central London. The announcement came just hours after the health secretary, Sajid Javid, rejected calls for tougher restrictions on daily life.

The health security agency said the individual had spent time in the Westminster section of London, but was no longer in the country, and that contact tracing was being performed. It said the case was linked to travel in southern Africa.

Dr. Jenny Harries, chief executive of the agency, said it was “very likely’’ there would be more cases in the coming days.

On Saturday, a day after the government learned of the first two cases, Prime Minister Boris Johnson said that masks would be mandatory on public transportation and in shops in England starting on Tuesday. Tighter testing rules for travelers arriving from abroad would also go into effect that day.

But the government has rejected the idea of ordering people to work from home where possible, introducing vaccine passports in England or requiring masks in restaurants. “This is about taking proportionate action against the risks we face,” Mr. Javid told the BBC on Sunday, speaking before the third case was confirmed.

Britain began suspending flights from six southern African nations on Friday, but some travelers had already arrived in London by the time the measure took effect.

Mr. Javid acknowledged that passengers landing on Friday were not tested at the airport and were able to leave as usual, including by public transport. He said that all travelers who had arrived from southern Africa within the past 10 days were being contacted and asked to take tests.

“We could not have acted more swiftly,” he said.

By contrast, in Amsterdam, Dutch health officials tested more than 500 passengers who arrived on Friday on two flights from South Africa. Those who were negative were allowed to leave the airport and quarantine at home, or to continue their journeys.

Mr. Javid also urged Britons to quickly get booster shots and said he expected advice “imminently” from scientific experts on expanding the scope of the country’s vaccine program, especially with regard to boosters.

Such measures would, he added, help to “protect the progress we have made so we can continue to look forward to Christmas with family and friends.”