More than 100 politicians from 24 different countries, including the UK, the US and the EU, have written a joint letter condemning China over the "arbitrary detention and unfair trial" of Jimmy Lai, a tycoon and pro-democracy campaigner.
The parliamentarians, led by senior British Conservative MP Alicia Kearns, are "urgently" demanding the immediate release of the 77-year-old British citizen, who has been held in solitary confinement at a maximum security prison in Hong Kong for almost four years.
The letter - which will be embarrassing for Beijing - was made public on the eve of Mr Lai's trial resuming and on the day after British Prime Minister Keir Starmer met Chinese President Xi Jinping on the sidelines of a G20 summit of economic powers in Brazil.
It also comes as Hong Kong jailed 45 pro-democracy activists.
The group of politicians, who also include representatives from Canada, Australia, Spain, Germany, Ukraine and France, said Mr Lai's treatment was "inhumane".
"He is being tried on trumped-up charges arising from his peaceful promotion of democracy, his journalism and his human rights advocacy," they wrote in the letter, which has been seen by Sky News.
"The world is watching as the rule of law, media freedom and human rights in Hong Kong are eroded and undermined.
"We stand together in our defence of these fundamental freedoms and in our demand that Jimmy Lai be released immediately and unconditionally."
Sir Keir raised the case of Mr Lai during remarks released at the start of his talks with Mr Xi on Monday - the first meeting between a British prime minister and the Chinese leader in six years.
The prime minister could be heard expressing concerns about reports of Mr Lai's deteriorating health. However, he did not appear to call for his immediate release.
Ms Kearns, the MP for Rutland and Stamford in the East Midlands, said the meeting had been an opportunity to be unequivocal that the UK expects Mr Lai to be freed.
"Jimmy Lai is being inhumanely persecuted for standing up for basic human values," she said in a statement, released alongside the letter.
"He represents the flame of freedom millions seek around the world.
"We have a duty to fight for Jimmy Lai as a British citizen, and to take a stand against the Chinese Community Party's erosion of rule of law in Hong Kong.
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"This letter represents the strength of international feeling and commitment of parliamentarians globally to securing Jimmy Lai's immediate release and return to the UK with his family."
Mr Lai was famously the proprietor of the Chinese-language newspaper Apple Daily in Hong Kong, which wrote scathing reports about the local authorities and the communist government in mainland China after Britain handed back the territory to Beijing in 1997.
The tabloid was a strong supporter of pro-democracy protesters who took to the streets of Hong Kong to demonstrate against the government in 2019.
But the media mogul was arrested the following year - one of the first victims of a draconian new security law imposed by the Chinese Communist Party.
His newspaper was closed after his bank accounts were frozen.
Mr Lai has since been convicted of illegal assembly and fraud. He is now on trial for sedition over articles published in Apple Daily.
(c) Sky News 2024: Over 100 politicians from multiple countries condemn China over detention of tycoon Jimmy Lai