Hong Kong's top court on Thursday reinstated activist Chow Hang-tung's conviction for inciting others to join an unauthorised assembly in 2021 marking June 4.

The former vice-chairwoman of the now-defunct Alliance in Support of Patriotic Democratic Movements in China was initially jailed for 15 months after a magistrate found that her writing on social media and in a newspaper had encouraged people to join the banned gathering.

She took the case to the Appeal Court, which quashed her conviction in late 2022 by ruling that the legality of the police's ban could not be established.

The Department of Justice (DoJ) then took the case to the Court of Final Appeal, arguing that the defendant couldn't challenge the validity of the gathering ban during criminal proceedings when it had been upheld by the appeal board on public meetings.

The judges at the top court were split on this matter. Chief Justice Andrew Cheung and permanent judge Johnson Lam sided with the DoJ, agreeing that the only way to challenge the ban was by way of a judicial review.

Cheung wrote that if a defendant is allowed to re-open the question of a gathering ban's validity when it had been upheld by the board, "this would certainly weaken the authority of a prohibition and might encourage people to hold and attend the public meeting despite the prohibition".

However, the three other judges, led by permanent judge Roberto Ribeiro, said Chow was able to challenge the police ban during her criminal proceedings.

Despite the differing findings on this point, these three judges also agreed that the Appeal Court was wrong to conclude that the police chief should have taken the initiative to come up with conditions to allow the June 4 gathering to take place.

This meant all five judges agreed that Chow's legal challenge had failed, and therefore her acquittal has been overturned and her sentence reinstated.

Chow has been remanded in custody for more than two years over a national security case involving the alliance.