The 10 officials attended the birthday party of Witman Hung Wai-man, principal liaison officer for Hong Kong, Shenzhen Qianhai Authority, on Monday. A guest at the party tested positive a day after the event when she was quarantined in Penny's Bay as a close contact of an infected family member.

In a press conference on Thursday evening, Lam said Tsui is likely to be considered a close contact among the attending officials and expects to be quarantined. She said that the case of immigration chief Au Ka-wang has yet to be determined.

Another eight officials arrived at the banquet before the woman who tested positive. They didn't stay and just briefly said hello.

They are:

• Secretary for financial services and treasury Christopher Hui Ching-yu, his undersecretary Joseph Chan Ho-lim

• Police commissioner Raymond Siu Chak-yee

• ICAC commissioner Simon Peh Yun-lu

• Undersecretary for home affairs Jack Chan Jick-chi

• Undersecretary for innovation and technology David Chung Wai-keung, political assistant Lilian Cheong Man-lei

• Undersecretary for constitutional and mainland affairs Clement Woo Kin-man.

"I am disappointed, especially in Casper Tsui," Lam said, adding she has now reminded colleagues not to attend banquets.

Asked if she should bear the responsibility for the officials' conduct, Lam said she shares part of the responsibility as the no. 1 person in charge of Hong Kong's anti-epidemic measures.

She said she would take appropriate action against individual officials without naming the specifics.

If officials ignore health advice, they cannot serve as good examples for the public, she said.

Sources said former customs commissioner Hermes Tang Yi-hoi also went to the banquet.

A picture circulates among politicians, showing Hung singing without a mask while putting his hands around the shoulder of unmasked guest Ellen Tsang Fung-chu - the former chairwoman of the Y.Elites Association and a member of the Election Committee.

Three lawmakers also confirmed their attendance, including Duncan Chiu Tat-kun of the Information Technology functional constituency who tested negative. Benson Luk Hon-man of the Election Committee constituency did not update his test result as of 9pm Thursday.

Another lawmaker Rock Chen Chung-nin of the Election Committee constituency said he and a staff member attended the banquet. He also said the staff member had, on the next day, returned to the Legislative Council Complex and his office at Citic Tower.

The Legislative Council Secretariat on Thursday said they were notified of the situation, and relevant lawmakers and their staff members will not be able to return to the Legislative Council Complex and their offices at Citic Tower before having obtained negative Covid results.

It also noted that floors where the lawmakers had stayed at the Legco complex and Citic Tower, will be disinfected today.