Hong Kong's top court has overturned the acquittal of a former lawmaker for revealing that a police officer who was on duty during clashes in Yuen Long amid the 2019 protests was facing an anti-graft probe.

Ruling in favour of the government, the Court of Final Appeal restored Lam Cheuk-ting's conviction and sentence on Tuesday.

In 2022, the former lawmaker was sentenced to four months in prison for disclosing an inquiry by the Independent Commission Against Corruption (ICAC) into misconduct allegations against a police officer, Yau Nai-keung.

A High Court judge later quashed Lam's conviction on three counts of disclosing probe details under the Prevention of Bribery Ordinance.

But the Department of Justice was granted permission to take the appeal against the former lawmaker's acquittal to the top court.

The Court of Final Appeal's ruling centred on two possible interpretations of a section under the ordinance, which stipulates protections against the disclosure of bribery and corruption probes.

The ordinance bans anyone with knowledge of an ongoing corruption investigation from disclosing the identity of the person involved, or the fact that the individual is subject to a probe.

One interpretation, the bench said, adopted "a literal approach" that confined protections against the disclosure of bribery and corruption probes.

The other, which read the ordinance contextually and purposively, required "nothing more than the disclosure of the existence of an investigation that is underway".

The court's decision was split three against two, with the majority – Chief Justice Andrew Cheung, Justice Roberto Ribeiro and non-permanent judge James Allsop – ruling that Lam's disclosure of a misconduct probe fell within the scope of the anti-graft ordinance.

"Their Lordships preferred the second construction as it better reflected the statutory intent to preserve the efficacy or integrity of ICAC investigations into corruption offences and to protect the reputation of subject persons," the judgement said.

Such an interpretation was preferable because "either the disclosure of fact or details may prejudice the investigation," Ribeiro wrote.

Echoing his fellow judge, Cheung said simply revealing the fact that an individual was under investigation would amount to a prohibited disclosure.

Judges Joseph Fok and Johnson Lam disagreed, saying the law specifically refers to the disclosure of someone who's under a corruption investigation.

Noting a previous amendment of the ordinance that narrowed the scope of law, Fok said the purpose of the current section was to protect the integrity of a bribery probe "rather than an ICAC investigation in general."