A High Court judge has ordered a law prohibiting people from using public toilets designated for the opposite sex to be struck down, ruling in favour of a transgender man who challenged the city's public convenience regulations.

In a landmark ruling on Wednesday, judge Russell Coleman said "drawing the line" of a person’s biological sex at birth creates a disproportionate and unnecessary intrusion into privacy and equality rights, and contravenes articles of the Basic Law and Hong Kong Bill of Rights.

However, Coleman suspended his order for a year to give the government time to come up with an approach, noting that officials might feel content to "let the criminal offence go".

"I suppose the view could be taken that, as with other conveniences accessible by the public (but privately managed), there are other offences which can be used to deter and punish improper conduct," he wrote in a 51-page judgement.

"This is a matter of the line-drawing, which seems to me to be a question for the government or legislature to address."

Coleman also said it would not be appropriate for the courts to judge where to draw the line between a "female person" and a "male person", as it was a matter for legislation, "probably in the context of wider or inter-linked questions".

The legal bid was filed by a transgender man identified only as "K", who sought to amend the wording of the Public Conveniences (Conduct and Behaviour) Regulation, which bans people aged five or above from facilities allocated to their opposite sex.

At the time of bringing the application, K held a gender identity letter showing he was undergoing a period of "real-life experience", during which he sought to live consistently in his adopted gender.

The court heard that the applicant pressed for changes to the regulation, to let transgender people diagnosed with gender dysphoria and having a medical need to undergo "real-life experience" access public toilets corresponding to their identified gender.

The government had argued that public toilet access rules protect other users' rights, including privacy and security, as well as societal expectations.