Parliamentarians and foreign affairs experts from at least 22 countries and the European Parliament signed an open letter, expressing concern about the 76-year-old media tycoon's incarceration.

"His health is deteriorating. He has been held in solitary confinement in a maximum security prison in Hong Kong for almost four years. This is inhumane," they wrote, calling the charges against him "trumped up".

The letter's release came on the same day a Hong Kong court jailed all 45 defendants convicted in the city's largest trial under a sweeping national security law imposed by Beijing in 2020.

The separate charges against Lai, who founded the now-shuttered popular tabloid Apple Daily, revolve around the newspaper's publications, which supported the pro-democracy protests and criticised Beijing's leadership.

Lai, who is a British citizen, denies two counts of "conspiracy to collusion" and one count of "conspiracy to publish seditious publications".

But the parliamentarians said his trial was "tainted with unfairness", involving "hand-picked judges" and evidence allegedly obtained by torture.

"On the eve of the recommencement of his trial, we urgently demand Jimmy Lai's immediate and unconditional release," they added, warning China that "the world is watching as the rule of law, media freedom and human rights in Hong Kong are eroded and undermined".

British Prime Minister Keir Starmer raised Lai's case with Chinese President Xi Jinping when they met on the sidelines of the G20 summit in Brazil on Monday.

Some 30 signatories were from the UK, which handed back sovereignty of Hong Kong to China in 1997 but which has been increasingly critical of the authorities in the territory for its crackdown on political dissent.