Ma Chun-man -- known as "Captain America 2.0" for carrying a replica of the Marvel superhero's shield during democracy rallies in 2019 -- was imprisoned for "inciting secession" under a national security law imposed by Beijing in 2020.

He had hoped to receive a one-third sentence remission -- typically granted to inmates for good behaviour -- and be released in March.

But the passage of another national security law this year effectively banned remission for people convicted of national security crimes, and Ma's request for early release was denied.

Ma challenged the decision, but on Friday the court said the new system was "sufficiently precise and certain".

"There is no question of Mr Ma being subject to any additional or heavier penalty by operation" of the new rules, High Court judge Alex Lee ruled.

"The applicant's substantive judicial review is dismissed," said Lee, who is among a pool of jurists selected by Hong Kong's leader to hear security cases.

Both of Hong Kong's national security laws have been criticised by Western nations such as the United States for quashing dissent and curtailing freedoms in the city.

Hong Kong authorities have defended the laws as necessary to restore order following the huge and sometimes violent pro-democracy protests in 2019.

Under the second law, national security prisoners are now "not entitled to remission", city leader John Lee has said, with the rule change covering those already serving time.

An exception can be made only if Hong Kong's prison chief is satisfied that an early release would not be contrary to national security interests.

Ma's lawyers argued that the definition of "interests of national security" was too vague.

But government lawyers responded that "national security is really a constantly evolving concept... not capable of being inflexibly defined".

Ma, a former food delivery worker, was sentenced to nearly six years in jail after being found guilty of "inciting secession" by chanting Hong Kong independence slogans, an offence under the first national security law.

The sentence was reduced to five years after a successful appeal in 2022.