Her nomination had been invalidated by returning officer Franco Kwok Wai-fun, who found that Lau had not genuinely changed her previous position of advocating self-determination for Hong Kong, despite her claim that she had ditched the stance ahead of the polls.

Disqualified Hong Kong lawmaker Lau Siu-lai was denied chance to respond to allegations of not upholding Basic Law, judge says

In his ruling, Chow said “the unfairness of the situation is obvious”, adding: “The principle of natural justice requires that a person affected by an adverse decision ought generally to be given an opportunity to make representations prior to the making of the decision.”

Kwok’s counsel, Johnny Mok Shu-luen SC, had argued that the present case was exceptional because Lau had been disqualified – over improper oath-taking – from the very office with which the by-election was concerned.

Lau, who was elected in Kowloon West in the 2016 polls, was among six opposition lawmakers who were stripped of their seats by a court for their improper oath-taking.

The swearing-in saga had prompted a controversial interpretation of Article 104 of the Basic Law – the city’s mini-constitution – by China’s top legislature, which ruled that public officials should take their oaths “sincerely” and “solemnly” or face disqualification.

But the judge said he was unable to accept Mok’s argument because there was nothing in Article 104 of the Basic Law or its interpretation to support the claim.

Chow also observed that Lau’s earlier oath-taking case, which resulted in her disqualification, only concluded that she had clearly failed to comply with requirements for taking the Legco oath during her swearing-in ceremony on October 12, 2016. It was not a general finding against her ability to satisfy requirements, he said.

He further observed that Mok’s argument could lead to the “illogical” result that Lau would be qualified to stand for election in all other constituencies, except for Kowloon West, even though her fitness to serve as a lawmaker would be exactly the same.

“I consider it to be clear that there was a material irregularity in relation to the by-election,” Chow said. “I also find that Ms Chan was not duly elected.”

Labour Party stalwart Lee Cheuk-yan served as Lau’s backup candidate in the 2018 by-election following her disqualification, but he eventually lost the seat to Chan, who was backed by pro-establishment parties.

Lau on Thursday described the ruling as merely a “bitter victory”.

Even with the judge pointing out the procedural irregularity in her disqualification, she said: “The returning officers could still disqualify me in the next polls. The political rights of citizens are still not guaranteed.”

A Legco meeting on Thursday was temporarily suspended when the news broke as pan-democrats questioned if Chan was still allowed to speak.

Chan, who was allowed to continue taking part, said she was undecided over to whether to appeal

"My work has not been affected, my attitudes are the same," she said. “I will treat this as a small obstacle.”

The ruling is unlikely to significantly impact the 70-member legislature, which even without Chan will still be dominated by 42 pro-establishment legislators.

It is expected that her vacated seat will not be filled by another by-election as the current Legco term ends in two months’ time. A Legco election is scheduled on September 6.

Last year, a court overturned two similar disqualification bans imposed on Demosisto member Agnes Chow Ting and localist Ventus Lau Wing-hong.

That led to the ousting of Au Nok-hin, Chow's backup candidate, and NeoDemocrat Gary Fan Kwok-wai, who won by-elections 2018 in the Hong Kong Island and New Territories East constituencies respectively.

Au and Fan were later officially unseated after the Court of Final Appeal refused to let them challenge lower court rulings declaring their elections invalid.

Additional reporting by Sum Lok-kei