A Hong Kong appellate court has overturned fraud convictions against pro-democracy media tycoon Jimmy Lai, though he will remain in prison due to a separate 20-year sentence under the Beijing-imposed national security law.

Lai, founder of the now-defunct Apple Daily, had previously been sentenced to five years and nine months for fraud, but judges ruled prosecutors failed to prove deception beyond reasonable doubt.

His overall prison term may be slightly reduced, yet concerns remain that he could spend the rest of his life behind bars.

The case has drawn international criticism, with supporters arguing it reflects a decline in press freedom, while Hong Kong and Chinese authorities maintain the prosecutions uphold the rule of law, The Guardian has reported.

In a separate case, the father of US-based activist Anna Kwok was sentenced to eight months in prison under the national security law for attempting to access funds linked to her.