When asked whether the government should ban non-locals from entering the city, Dr Chuang Shuk-kwan, head of the centre’s communicable disease branch, said all the fresh infections were residents, except one – an Australian who had been to the United States and Portugal. He was transiting at the airport and sent to hospital after feeling unwell.

Chuang warned the city might record a high number of cases in the coming days. “As some of the cases had travel history a few days before … with the incoming number of residents returning to Hong Kong, we may see a high number of cases for at least two weeks or more,” she said.

The Hospital Authority said it might need to further cut non-urgent services to handle the increasing number of infections.

The jump in cases came as Hongkongers were warned against letting their guard down. “This is the worst time to relax because we’re at the highest risk since this began,” said Professor Gabriel Leung, dean of the University of Hong Kong’s (HKU) medical faculty and a member of Chief Executive Carrie Lam Cheng Yuet-ngor’s expert panel on the viral outbreak.

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“It is absolutely critical especially as we have this influx of Hong Kong returnees starting from a few days ago and continuing for the next few days. This is the highest risk and we must be extra vigilant.”

Leung said the large number of Hongkongers returning from overseas could spark a secondary wave of infections, spawning more local clusters. He warned that even a few patients spreading the disease could lead to large numbers of local clusters. He said such a scenario had recently played out in South Korea, which had more than 8,600 infections and close to 100 deaths.

Leung appealed to arriving passengers to be truthful with local authorities about their health status.

“Only with a very smooth and passenger-focused experience, would [returnees] be as forthcoming as they could, to not only protect themselves but protect the Hong Kong population,” he said.

Top microbiologist Ho Pak-leung, also of HKU, said the city should brace for as many as 200 imported cases of Covid-19 in the next two to three weeks, likening the local crisis to a war. Ho suggested isolating arriving travellers at hotels, otherwise they will be sharing bathroom facilities with others and risk infecting them.

“When one patient passes the virus to two people in the community to form an unknown number of hidden transmission chains, then it’s very likely that we will have 400 to 600 cases in the next two weeks,” he said. If that happened, then “Hong Kong is at war”.

One of the new cases on Friday involved a member of Pure Fitness gym in Central, where two others were earlier confirmed as infected. Another concerned a female Standard Chartered bank employee who attended a wedding banquet, as did three other confirmed patients, on March 14. The woman works at the Chung On Street branch in Tsuen Wan.

The company said the branch had been closed until further notice, and all staff there would go into 14-day self-quarantine. The bank said the infected patient went to work on Tuesday before taking sick leave and that it would inform clients who might have been in close contact with her.

More than 13,100 travellers – including 11,924 Hongkongers – arrived in the city on Thursday, after mandatory home-quarantine orders kicked in for all arrivals from foreign countries. That figure was 36 per cent lower than Wednesday’s count of 20,583

Leung suggested businesses such as restaurants slash opening hours and limit the number of patrons, while maintaining good hygiene. He said some countries had legally limited the opening times of places that might draw crowds, such as bars. “The government should consider if these stringent measures apply to Hong Kong,” Leung said, but these should be a “last resort”.

People delivering supplies to the elderly should avoid face-to-face contact all together, “however hard it may be”, he added. Although people might want to visit the elderly to lift their spirits, doing so could also risk infecting them, he warned. “What I do not want to see is love turning into harm.”

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Separately, health minister Sophia Chan Siu-chee told a panel meeting at the legislature that the government would consider barring entry to non-Hong Kong residents if it was deemed necessary, while calling on residents under home quarantine to practice self-discipline.

Some 33 arrivals at the Hong Kong airport were earlier screened on Friday and sent for testing at two new temporary facilities the Hospital Authority has opened to handle the influx of people returning from overseas. The facility at AsiaWorld-Expo tested 31, while the North Lantau Hospital handled two. The first group arrived from Southeast Asia and the other two from Europe and the United States.

Results of the first batch of nasal samples, taken around 11am, would be verified about eight hours later.

“For the laboratories, that is already very efficient but we will look to improve the logistics so that travellers may know their results earlier,” said Dr Larry Lee Lap-yip, deputy chief executive of Tin Shui Wai Hospital.

Additional reporting by Kanis Leung and Zoe Low