The High Court on Sunday rejected an application by pro-democracy lawmakers for an interim injunction against the mask ban imposed by Chief Executive Carrie Lam using emergency powers.
But the court will decide later this month whether to allow a judicial review into the CE's move, and any hearing could come at the same time.
The 24 pro-democracy lawmakers had argued that the ban was unconstitutional because Lam had bypassed the legislature to bring it in.
They issued a statement after the hearing, describing the way the High Court had handled the matter as "extremely rare".
They said the planned combined judicial review application bid and hearing shows the court agrees with their questioning of the constitutional basis for the government invoking the Emergency Regulations Ordinance.
They urged the government to enforce the anti-mask law with great restraint, before any court decision.
The lawmakers noted that some people have already been arrested for allegedly violating the new law and said if the government still treasures Hong Kong's rule of law, it should stop enforcing a mask ban which might be overturned.
They also expressed concern that the government might make use of the emergency powers again to introduce other new laws to strip citizens of their rights and freedom.
Sunday's was the second case over the mask ban in as many days, with the High Court also rejecting a bid for an urgent injunction on Friday night.
Anyone who violates the ban on wearing masks at any kind of protest, authorised or otherwise, risks a prison term of one year and a fine of up to HK$25,000.
Aside from protests, the police can also order people to remove face masks, with those refusing liable to six months in prison or a HK$10,000 fine.