The decision came after UK Prime Minister Boris Johnson on Monday delayed the launch of anti-epidemic measures by a week to December 28, despite a sharp rise of Omicron cases in the country.

During the week ended Monday, the UK recorded an average of 82,808 cases per day, up 62% from the previous week. On Monday, that figure hit 91,734, which reports say might include more than 10,000 Omicron cases.

Hong Kong’s new quarantine measure for UK arrivals was announced after France and Germany banned travelers from Britain over the weekend. Health experts said Hong Kong would inevitably see more imported Omicron cases in the coming weeks as the strain is becoming dominant several in Western countries.

Hong Kong’s quarantine rules have been dubbed the most stringent in the world as travelers must undergo 21-day isolation at designated hotels, with costs ranging from HK$530 (US$68) to several thousand dollars per night.

The city’s government said it would continue to follow mainland China’s “zero local infection” strategy and push for the resumption of quarantine-free travel across its border with the mainland. Beijing is expected to make a decision on reopening the border this week.

Hong Kong’s Department of Health said on Tuesday that it had recorded 13 more Omicron cases over the past three days, bringing the number of cases related to the highly contagious strain up to 27. Five of those came from the UK.

One 77-year-old woman who arrived from the UK tested positive for Omicron, though she had received a third jab of the Pfizer-BioNTech vaccine on November 3.

Infectious diseases expert Leung Chi-chiu predicted more Omicron cases would be discovered in Hong Kong in the coming weeks as people returned from overseas for Christmas and New Year, as well as Chinese New Year on February 1.

Leung said it was necessary to further tighten the city’s quarantine rules and require incoming flight crews, who could be exempted from 21-day isolation, to do more Covid tests.

On Monday evening, the Hong Kong government announced a series of measures to prevent the spread of Omicron, including a shortening of the lead time for specimen collection for pre-departure nucleic acid tests for people boarding flights to Hong Kong from 72 hours to 48 hours, with effect from Friday.

The UK was put today (December 21) on a list of “specified places for enhanced quarantine and surveillance measures on Omicron variant”, along with the US and a dozen African countries. Incoming travelers will need to be isolated at Penny’s Bay Quarantine Center for four days before they can spend their remaining 17 days at quarantine hotels.

Previously, people coming from places on this list had been required to stay at the quarantine camp for seven days after arrival, but the government on Tuesday shortened the period to four days.

Ho Pak-leung, head of the University of Hong Kong’s Center for Infection, said Tuesday that as the incubation period of the Omicron variant was about two to three days on average, more than half of the Omicron cases could be identified at Penny’s Bay Quarantine Center.

Ho said it was good to send newly arrived travelers from highest risk countries to the quarantine camp, which he said has better ventilation than quarantine hotels.

Benjamin Cowling, an epidemiologist at the University of Hong Kong, said earlier this month that putting people into Penny’s Bay Quarantine Center for the first seven days might not be sustainable because it was not suitable for some travelers such as bankers and company executives who planned to stay at luxury hotels upon arrival.

He said Hong Kong might have to consider constructing a special facility for arrivals to spend the first seven days in quarantine.

Many people who stayed at Penny’s Bay Quarantine Center have complained about the food and environment. In May, 40 people suffered from food poisoning, forcing the government to change the food supplier.

If more countries are included on Hong Kong’s list of “specified places for enhanced quarantine,” more overseas Hong Kong residents will likely push back plans to return to avoid staying at the camp, commentators said. Such a trend will further discourage international travel to Hong Kong, reducing the Asia financial city’s openness, they said.

The government said Monday that if four or more passengers were confirmed with Covid-19 upon arrival in Hong Kong from the same flight route within seven days, that route would be suspended for 14 days.

On Tuesday, it announced a ban on the landing of a passenger flight from Doha operated by Qatar Airways in Hong Kong from Wednesday to January 4.

It said passenger flight QR818 operated by Qatar Airways arriving in Hong Kong from Doha had five passengers who were found to be infected after tests on arrival between December 14 and Monday.

Prior to this, Cathay Pacific flights from New York were banned until January 2 after CX845 from New York last Sunday had three passengers confirmed with Covid-19 on arrival, likewise triggering the circuit-breaker.