The UK government and international rights groups have condemned the conviction of former pro-democracy newspaper owner and British citizen Jimmy Lai in Hong Kong on national security charges.

On Monday, Lai, 78, was found guilty in West Kowloon district court on one count of conspiracy to publish seditious publications and two counts of conspiracy to foreign collusion. The charges were brought under the city’s punitive national security law (NSL), introduced in 2020, and a British colonial-era sedition law that has been used in recent years by authorities.

The pro-democracy activist and former media mogul now faces spending the rest of his life in prison.

Yvette Cooper, the British foreign secretary, said: “The UK condemns the politically motivated prosecution of Jimmy Lai that has resulted in today’s guilty verdict.

“Jimmy Lai has been targeted by the Chinese and Hong Kong governments for peacefully exercising his right to freedom of expression … We continue to call for Mr Lai’s immediate release.”

Sebastien Lai, the son of jailed media tycoon Jimmy Lai. Photograph: Toby Melville/Reuters

Lai is the founder of Apple Daily, a hugely popular pro-democracy newspaper in Hong Kong that was forced to close in 2021. Lai was arrested in 2020 and has spent more than five years behind bars, in a lengthy trial that has been delayed several times.

Sarah Brooks, the China director for Amnesty International, said: “The conviction of Jimmy Lai feels like the death knell for press freedom in Hong Kong, where the essential work of journalism has been rebranded as a crime.

“This verdict shows that Hong Kong’s so-called national security laws are not in place to protect people, but to silence them. It should also serve as a warning to all people doing business in Hong Kong: that pursuing opportunities in the city comes with severe legal risks.”

Hong Kong media tycoon Jimmy Lai convicted of national security offences – video

Speaking to reporters in London on Monday, Lai’s son Sebastien Lai said while the guilty verdict was not a surprise, it was “still painful”.

“It signifies a Hong Kong that has completely changed,” Sebastien said.

Sebastien and Lai’s international legal team called on the UK government to make the release of Lai a pre-condition for closer relations between London and Beijing. The UK prime minister, Keir Starmer, has called for Lai’s release and has raised the case directly with China’s president, Xi Jinping. Starmer is expected to visit Beijing in January.

Caoilfhionn Gallagher KC, the head of the international legal team, said that there was “no use shouting into the wind” and statements in support of Lai needed to be backed up by concrete action. “China needs to see there are real consequences” for Lai’s prosecution, Gallagher said.

Thibaut Bruttin, the director of Reporters Without Borders, said: “We are outraged that Jimmy Lai, Hong Kong’s symbol of press freedom, has been found guilty on trumped-up national security charges. This unlawful conviction only demonstrates the alarming deterioration of media freedom in the territory: make no mistake: it is not an individual who has been on trial –it is press freedom itself, and with this verdict that has been shattered.”

Cooper also called for Lai to be given full access to independent medical professionals. There have been increasing concerns about the 78-year-old’s health. Lai has diabetes and has been kept in solitary confinement for more than 1,800 days. Earlier this month his family said that Lai had lost a significant amount of weight, his nails were discolouring and falling off, and his teeth were rotting. The closing arguments in Lai’s trial were delayed at one point because he was suffering from heart palpitations.

Claire Lai, Lai’s daughter, said: “Having spent the last few years in Hong Kong, I have personally witnessed my father’s rapidly failing health. He is 78 and has spent five years in terrible conditions, we are worried about how much more he can bear.”

China’s foreign ministry said: “We urge the relevant countries to ... not make irresponsible remarks on the trial of judicial cases in Hong Kong and not to interfere in Hong Kong’s judiciary or China’s internal affairs in any form.”