Sebastien Lai said a fresh diplomatic push was now needed to free the 77-year-old pro-democracy activist, who holds British citizenship and has been kept behind bars in Hong Kong since December 2020.

Jimmy Lai's lawyers have complained about his conditions in detention and say they are increasingly worried about his health.

"I don't want my father to die in jail," Sebastien Lai told AFP in London. "I need to meet with the prime minister imminently."

This week Lai concluded more than 50 days of courtroom testimony as he fights charges of foreign collusion under Hong Kong's national security law. The law was imposed by China to quell dissent after pro-democracy protests in 2019.

Lai is the founder of the now-shuttered Chinese-language tabloid Apple Daily, which supported the protests, and he is one of the most prominent figures to be prosecuted under the law.

The collusion offence carries a maximum penalty of life in prison, while Lai also faces a charge related to "seditious publications".

"It is desperate now," Sebastien Lai told journalists at a briefing in London organised by the Reporters Without Borders (RSF) rights group. "I really don't know how long my father has."

UN rights experts have called for Lai's release, as have Western governments including Britain.

A UK government spokesperson said: "British national Jimmy Lai's case is a priority."

"We continue to call on the Hong Kong authorities to end their politically motivated prosecution and immediately release Jimmy Lai."

- A 'pivotal moment' -

Last year Starmer raised Lai's case with Chinese President Xi Jinping and has said securing his freedom was a "priority" for the government.

But in London, Lai's legal team said more urgency was needed.

Lawyer Caoilfhionn Gallagher said delays in proceedings coupled with Lai's age, health and conditions of detention could lead to "a British national dying in prison for being a journalist and for standing up for democratic values.

"That's why there's such an urgency today to actually getting on with sitting down to talk to the government," she told journalists.

RSF's UK director Fiona O'Brien said Lai's plight had reached a "pivotal moment". It was, she said, "a case about the values of press freedom for which he has fought his whole life".

But Sun Qingye, the deputy chief of Beijing's national security apparatus in Hong Kong, on Friday dismissed the idea of freeing Jimmy Lai. It was wrong to put pressure on judges, he added.

"This matter must be dealt with according to law," Sun told reporters on the sidelines of Beijing's annual political gathering.

"Why grant a pardon? What would be the reason?"

In Hong Kong, Lai's lawyers concluded their defence on Friday, a day after he wrapped up his testimony. The case was adjourned to July 28 for closing arguments.