The government has won an appeal against the acquittal of four people accused of rioting in Wan Chai in 2019, with a court ordering they be put on trial again.

The prosecution of social worker Jackie Chen came to an end in 2020 when District Court judge Sham Shiu-man ruled she had no case to answer. Chen had seven co-defendants who were all eventually found not guilty.

But the Court of Appeal said on Friday that Sham had speculated in favour of the defendants, and his ruling regarding Chen was "wrong in law and perverse".

It said the judge had only considered whether the social worker's actions had breached the peace, without giving thought as to whether she had stayed at the scene of the riot to encourage others to take part and therefore jointly committed the offence with them.

Sham's wrongful speculation included that the defendants could have worn masks and other gear merely for their own protection, and that they could have simply wanted to witness what was happening at the scene, the appeal court said.

Chen and three of the other defendants now face a retrial before a new judge.

The Department of Justice's appeal against the acquittals of the other four people in the case was dismissed because they had left Hong Kong and could not be served with the relevant paperwork.