Legal experts, meanwhile, had called for the government to scrap the ban no matter the outcome to avoid confusion, and said the ruling would not affect those wearing masks for health purposes.

In Thursday’s unanimous decision, High Court Chief Judge Jeremy Poon Shiu-chor and Justices Mr Johnson Lam Man-hon and Thomas Au Hing-cheung said the city’s chief executive required flexible powers to deal with exigencies “expeditiously and effectually”.

“If the Emergency Regulations Ordinance were held to be unconstitutional, it would leave a significant lacuna [blank space] in the law for dealing with emergency and public danger generally,” they said, adding that the city’s leader would be best placed to decide what a public danger was.

The Department of Justice said it would study the judgment in detail.

The Security Bureau would not say whether the mask ban would be repealed or reviewed, but added that incidents related to explosives and firearms in the city “continue to cause serious concerns”. It also stressed the mask ban was to “strengthen the deterrent effect against radical protesters”.

A police spokesman said the force would “strictly comply” with the judgment but acknowledged there was a need for members of the public to wear masks for health reasons protected under the regulation.

“Police officers will therefore handle incidents flexibly with regard to the actual circumstances,” he said.

Anti-mask law to quell Hong Kong protests ruled unconstitutional by High Court

But veteran activist “Long Hair” Leung Kwok-hung, one of those challenging the ban, said the definition of “public danger” was precisely what he hoped the Court of Final Appeal would now be able to clarify.

“The definition of ‘public danger’ is too wide, it will allow the chief executive to abuse his or her power,” he said outside the courthouse. Both Leung and 24 pan-democratic lawmakers behind a separate challenge have vowed to pursue an appeal.

Meanwhile, pro-establishment lawmaker Priscilla Leung Mei-fun, an adviser to Beijing as a member of the Basic Law Committee, welcomed the decision, saying it was important the chief executive maintained the discretionary power to issue emergency regulations.

Although the bill was withdrawn in September, the movement had by then morphed from peaceful marches into citywide protests that routinely resulted in vandalism and violent confrontations with police.

The government introduced the mask ban, designed to prevent protesters from hiding their identities, by invoking the colonial-era Emergency Regulations Ordinance (ERO), arguing the unrest amounted to a “public danger”.

But the move, which banned face coverings at not just illegal assemblies but also lawful ones, sparked fears over declining civil liberties, prompting judicial challenges.

On Thursday, the three presiding justices concluded a fine balance could be struck between individual rights and the public interest by limiting the ban to unlawful and unauthorised assemblies.

Government starts appeal of court’s decision to declare mask ban unconstitutional

They noted that at the height of last year’s anti-government protests, some assemblies, while peaceful at the outset, had devolved into riots.

Banning masks was necessary, as while some protesters did not take part in the violence, they remained at the scene to provide moral support to the perpetrators, they said.

The judges added, however, that police should first order these people to leave – and give them ample opportunity to do so – before enforcing the law.

Authorised assemblies should be exempted entirely, the judges ruled, as they had been approved by police, who could issue an order of dispersal should they grow out of control.

The justices also argued the ERO did not give the chief executive “unfettered and unlimited” power, but rather represented a delegation of Legislative Council authority, much as was done in the days of city governors.

They further said that just as the Basic Law gave the central government the right to apply national laws in Hong Kong in times of war or turmoil, the local government had the right to invoke the ERO in times of emergency.

What happens if Hong Kong court upholds mask ban amid coronavirus pandemic?

Challenges filed by Leung Kwok-hung and the pan-democratic lawmakers in October argued the ban was damaging to freedom of expression and assembly, and that the ERO, which gave the government the power to pass subsidiary legislation without Legislative Council scrutiny, was inappropriate.

A month later, the High Court ruled both the ban and this particular use of the ERO were unconstitutional, saying the “near blanket prohibition” on face coverings and the force’s unfettered power to enforce it were not “reasonably necessary”.

The decision infuriated Beijing, which said Hong Kong courts did not have the power to declare a law incompatible with the Basic Law, prompting the appeal from the Department of Justice.

On Thursday, the appeal court reaffirmed the city’s mini-constitution did indeed give it the power to make those distinctions, while arguing that in no way diminished the authority of the National People’s Congress Standing Committee.

Since the ban was put in place, 682 people have been arrested for wearing masks, with 61 prosecuted, according to police.