Secretary for Home Affairs Caspar Tsui Ying-wai, the most senior official involved in the scandal, on Saturday apologised again and said he “should have turned down the invitation”.

“I failed the citizens’ trust and expectation and damaged the overall anti-pandemic efforts,” he wrote on his Facebook page. “I will not make the same mistakes. I am also willing to make things right, and accept the treatment I deserve.”

Tsui, Director of Immigration Au Ka-wang and political assistant to the secretary for development Allen Fung Ying-lun – all of whom were present at the party after 9.30pm, when the confirmed coronavirus patient showed up – are now in quarantine at the government’s Penny Bay facility.

According to a government spokesman, 11 other senior officials, who were released from the camp on Saturday after a separate suspected case detected at the party was declared a false positive, had to undergo home isolation using their own vacation leave and conduct regular testing “to minimise the risk of possible transmission”.

The 11 can only resume duties after next Friday if all their test results are negative.

“After the investigations are completed, the chief executive will take appropriate actions against the relevant officials in an impartial manner,” the spokesman said.

Health authorities on Sunday revealed a new preliminary-positive case involving a 43-year-old woman who worked in Cable TV Tower in Tsuen Wan and attended the same party. Her case did not necessitate any changes to the quarantine orders, as the uninvited guest had arrived at the venue after 9.30pm when most officials and lawmakers reported that they had left.

Three days after city leader Lam ordered an investigation into the conduct of the officials, a source close to Beijing said the central government had asked her to “take swift action” against the officials who ignored the local authorities’ own warning to avoid large gatherings.

Most of the officials involved are political appointees, including Tsui and the chiefs of the police, immigration and anti-graft agencies, who are under non-civil service contracts and not subject to established disciplinary mechanisms applicable to civil servants.

According to the Civil Service Bureau, civil servants are issued warnings for minor misconduct. Formal disciplinary action or a criminal conviction is considered for serious misconduct.

The result of the probe, led by director of the Chief Executive’s Office Eric Chan Kwok-ki, is being closely watched.

Hong Kong Covid-19 rules may have been broken at birthday party: lawyers

Lau Siu-kai, vice-president of Beijing-based think tank the Chinese Association of Hong Kong and Macau Studies, warned that prompt action against possible violations at the party was necessary to assuage public anger.

“The scandal has nothing to do with national security. Citizens, who were already affected by the tough social-distancing measures, might take this chance to amplify their anger towards the government or certain officials,” he said.

“If Lam’s government reacts sloppily again, this would lead to a serious blow to the credibility of governance.”

Senior counsel Ronny Tong Ka-wah, one of Lam’s advisers, said if officials’ attendance at the banquet constituted any breach of discipline, the city leader should consider imposing punishments, including demotion, pay cuts or dismissal, to demonstrate that the political accountability system “existed in reality and not just in name”.

Under the system, all bureau chiefs are appointed by Beijing on recommendation from the chief executive.

Covid-19 party scandal: Hong Kong leader suspends 13 senior officials

Regina Ip Lau Suk-yee, also one of Lam’s advisers, called for prompt investigations into the alleged violations, adding that the city leader had the power to consider what punishments were appropriate.

The partying scandal was exposed days after authorities announced strict measures to curb the spread of the Omicron coronavirus variant, including a ban on evening dine-in services at restaurants.

While senior officials and lawmakers had drawn ire for attending the party, some pro-establishment politicians and news outlets pointed fingers at Cathay Pacific for triggering a fresh wave of infections after aircrew flouted pandemic rules and caused a community cluster of the Omicron variant.

Tam Yiu-chung, the sole local delegate to the country’s top legislative body, said the top priority now was to “plug loopholes” regarding aircrew members. Pro-Beijing newspaper Ta Kung Pao, in an editorial on Sunday, reprimanded the airline for its “serious problems” in its management which worsened the pandemic situation, and called for criminal investigation against the company.

Anger, apologies as top Hong Kong officials caught in Covid-19 party scandal

But Tong said there were no legal means to go after the embattled airline as entities could not sue a business for “pure economic loss”.

“Unless you can prove there was systemic negligence that resulted in damage to life and property … But that was very unlikely in this case, as the exemptions were granted by the government,” he said, adding that authorities had made a tough decision in balancing the Covid-related risks and economic interests.

The latest health crisis had also prompted calls to adopt citywide testing but respiratory medicine specialist Dr Leung Chi-chiu said requirements for effective testing included restricting residents’ movements, which was difficult for Hong Kong to follow.

“We need at least three days to finish citywide testing. Meanwhile, we have to ask 7.5 million citizens to stay at home. The economy will collapse before the virus is dead,” he said.

The city’s confirmed Covid-19 case tally stood at 12,935, with 213 related deaths.