A Hong Kong cabinet minister resigned on Monday for attending a birthday party alongside dozens of officials and lawmakers just days after the city's government called on people to avoid large gatherings to fight a coronavirus outbreak.

"I have today tendered my resignation to the Chief Executive and intend to leave the post today," Home Affairs Minister Caspar Tsui said in a statement.

"As one of the Principal Officials taking the lead in the anti-epidemic fight, I have not set the best example during the recent outbreak," he added.

Tsui's resignation is a blow to the administration of Chief Executive Carrie Lam whose time in office has been marked by huge pro-democracy protests and a subsequent crackdown on dissent that has transformed Hong Kong.

The January 3 birthday bash at a tapas restaurant for Witman Hung, a member of China's top lawmaking body, became a source of embarrassment for Lam as her government pursues a strict "zero-Covid" policy similar to Beijing's.

The guest list emerged after health authorities traced an infected person to the party.

Among the more than 200 people present were over a dozen top officials -- including the city's police, immigration and anti-corruption chiefs -- as well as 20 lawmakers.

While the party was not illegal at the time under Hong Kong's strict social distancing rules, the city's health chief had warned people three days earlier to avoid large groups.

Tsui's conduct "brought the Hong Kong government to disrepute, and it has caused a negative perception" to the public, Lam said in a press conference Monday, hours after his statement was published.

She issued warnings to other officials who attended the party but deemed Tsui to be the most egregious offender given his key role in the city's anti-epidemic planning efforts.

Hong Kong has been keeping the coronavirus at bay through some of the world's strictest Covid rules
Hong Kong has been keeping the coronavirus at bay through some of the world's strictest Covid rules AFP / Peter PARKS

Lam added that Beijing has been briefed about her government's investigation into the January 3 birthday bash.

Tsui, 45, was a rising political star groomed by Hong Kong's largest pro-Beijing party, the DAB, which has further thrived under a new "patriots only" political system imposed by China that has criminalised much of the traditional pro-democracy opposition.

Hong Kong has been keeping the coronavirus at bay through some of the world's strictest Covid rules including long mandatory quarantines for all arrivals, restrictions on public gatherings and compulsory mask-wearing in public apart from when eating.

Photos from the party leaked to local media showed attendees singing karaoke and posing for group pictures without wearing masks.

Since the tapas party, multiple other clusters of the Omicron and Delta coronavirus variants have been detected, leading to the reimposition of harsher restrictions including the closure of gyms and bars and forcing restaurants to only serve takeout in the evenings.

Hong Kongers do not get to choose their leaders -- the source of years of pro-democracy protests -- and it is rare for cabinet members to resign.

While campaigning for the top job, Lam famously said she would step down if she lost popular support. Her administration has had consistently low approval ratings, dipping as low as 18 percent during democracy protests in 2019.

A new chief executive will be chosen by a small committee of Beijing loyalists in late March.

Lam has not said whether she will seek a second term and Beijing has yet to give a nod to its preferred candidate.