The same judge had previously ruled in favour of Demosisto member Agnes Chow Ting, who filed a similar petition complaining that she was not given an opportunity to respond after the returning officer disqualified her from contesting a by-election for a Hong Kong Island seat.

In that case too, the judge decided that the winner – pro-democracy lawmaker Au Nok-hin, who ran in Chow’s place – was not duly elected.

“I see no reason to differ from the conclusions that I reached in Chow’s case,” the judge wrote. “The returning officer ought to have given Mr Lau an opportunity to respond … and her failure to do so amounted to a material irregularity in the by-election.”

Lau, who was not in court on Friday, said in a statement that his successful petition was “definitely not a victory”.

“True victory comes the day when individuals of all kinds of political views in Hong Kong can stand in universal and equal suffrage and speak for the people,” he said. “To achieve this, an election petition is far from enough.”

He also called on the pan-democratic camp to stay united despite the possible loss of two Legco seats, now that lawmakers Au and Fan have been declared not duly elected.

Lawmaker Fan, who went to court with his lawyers to collect the judgment, said the disqualification of elected lawmakers and election candidates contributed to the ongoing political turmoil in Hong Kong.

Acknowledging that he might lose his seat, he said he would study the judgment and monitor Agnes Chow’s case to decide whether to appeal, but stressed he would continue to fight for democracy, human rights and the rule of law.

“An individual seat is nothing compared to Hong Kong’s core values and [the principle of] ‘one country, two systems’,” he said.

Agnes Chow and Lau stepped forward as candidates in last year’s by-elections that were called after pro-democracy lawmakers were ousted from the Legco for taking their swearing-in oaths improperly.

Lau submitted his papers to run as an “independent localist” to replace the unseated Sixtus Baggio Leung Chung-hang of the same camp.

But returning officer Amy Chan Yuen-man declared Lau’s nomination invalid because she was not satisfied that he genuinely and truly intended to uphold the city’s mini-constitution, the Basic Law, and pledge allegiance to the Hong Kong Special Administrative Region.

That was despite Lau declaring publicly that he no longer supported independence for Hong Kong, after learning from his experience working with a Sha Tin district councillor that most people were against the notion.

His lawyer, Senior Counsel Jin Pao, argued that depriving Lau of the right to stand for election was more serious than what happened in Agnes Chow’s case, because the returning officer formed the view that Lau was disingenuous and did not even consider whether he should be given a chance to respond.

Additional reporting by Sum Lok-kei