The daughter of Jimmy Lai, the pro-democracy tycoon jailed in Hong Kong, has expressed concerns about her father's deteriorating health in prison, saying that his fingernails "sometimes fall off" and his teeth are rotting.

Lai, 78, has been detained since December 2020 and faces life in prison. Earlier this month he was convicted of colluding with foreign forces under the city's controversial national security law (NSL).

Hong Kong and Chinese authorities have denied that Lai has been mistreated in prison, saying he is in "good health".

The BBC has seen a letter written by Lai's family urging UK Prime Minister Keir Starmer call for Lai's release when he meets Chinese President Xi Jinping next month.

His daughter Claire Lai, who's in exile in London, told the BBC that she fears she "may never see him again".

"Of course I worry that my father is going to be a martyr, and I don't want him to be a martyr. But I would not be here today, I would not be speaking out if I didn't think this was his best chance of reuniting with our family."

She described her father as an "extremely robust and strong man" when he went to prison, but has over the last year he has "lost a significant amount of weight".

"He is diabetic, he has heart issues which he never had in the past. He has fingernails that turn purplish grey and sometimes fall off. He has teeth that are rotting.

"He has back and waist pains. Some days it's painful for him to stand. Sometimes he can't stand. And some days he can't even get out of bed."

Lai's family has on multiple occasions flagged concerns about his deteriorating health. Lai's son, Sebastien, told the BBC earlier this year that his father's "body is breaking down".

A spokesperson for the Commissioner's Office of the Chinese Foreign Ministry in Hong Kong stated earlier this month that Lai's "lawful rights and interests have been fully protected, and he is in good health".

They added that "a safe, humane, appropriate, and healthy custodial environment in accordance with the law" has been provided for Lai.

Lai, a UK citizen, is the most prominent person to be charged under the NSL, introduced in 2020 in response to massive pro-democracy protests in Hong Kong.

Beijing defends the law as necessary to maintain stability in Hong Kong, but critics say it has effectively outlawed dissent.

Lai was accused of lobbying foreign governments to impose sanctions on Hong Kong and China. He was also found guilty under a separate colonial-era law of publishing seditious material in his now-defunct Apple Daily newspaper.

After Lai's conviction earlier this month, the UK condemned it as "politically motivated persecution", saying that he had been "targeted... for peacefully exercising his right to freedom of expression".

"The UK has repeatedly called for the National Security Law to be repealed and for an end to the prosecution of all individuals charged under it," the Foreign, Commonwealth and Development Office said.

In their statement earlier this month, Chinese authorities said Western attempts "to influence judicial verdicts through political pressure blatantly trample on the spirit of the rule of law and fully expose their hypocritical 'double standards'. The Chinese side firmly opposes this."

Starmer's trip to China in late January 2026 will be the first by a UK Prime Minister since 2018. (BBC)