The 78-year-old founder of the now-defunct Apple Daily newspaper was found guilty in December on two counts of foreign collusion under a sweeping national security law imposed by Beijing, as well as one count of seditious publication.

British Prime Minister Keir Starmer said he had raised the issue of Lai, a British citizen, during his meeting with Chinese leader Xi Jinping in Beijing last month, adding that the discussion was "respectful".

US President Donald Trump has also called for Lai's release.

Lai is set to hear his sentence at an hour-long hearing before three High Court judges from 10 am (0200 GMT), according to the judiciary.

He has already spent more than five years behind bars as the trial has progressed.

The Committee to Protect Journalists said in a statement before Monday's sentencing that Lai's trial "has been nothing but a charade from the start and shows total contempt for Hong Kong laws that are supposed to protect press freedom".

Reporters Without Borders said the mogul's sentencing "will resonate far beyond Jimmy Lai himself, sending a decisive signal about the future of press freedom in the territory".

Beijing has dismissed critics as smearing Hong Kong's judicial system, while Hong Kong authorities say Lai's case "has nothing to do with freedom of speech and of the press".

Around 70 people braved the cold to queue outside the West Kowloon court at daybreak, while dozens of journalists gathered outside the building's entrance.

"We hope that (Lai) can regain his freedom... I am worried, because of his advanced age," said activist Tsang Kin-shing, a member of the now-disbanded League of Social Democrats.

"Apple Daily has shut down and the media's mentality has changed completely," Tsang told AFP.

The mood outside court was tense as police deployed dozens of officers, an armoured car and a bomb disposal van to the area.

- Stiff penalties -

Long a thorn in Beijing's side, Lai was prosecuted under a Hong Kong national security law that was imposed by Beijing in 2020, a year after huge and sometimes violent pro-democracy protests in the finance hub.

The judges said in their verdict in December that Lai had "harboured his resentment and hatred of (China) for many of his adult years" and sought the "downfall of the Chinese Communist Party".

On Monday, they are expected to first decide whether Lai's collusion offences were "of a grave nature" and, if so, settle on a prison term of between 10 years and life.

The colonial-era offence of seditious publication comes with a maximum penalty of two years.

Lai will be sentenced alongside eight co-defendants, including six Apple Daily executives, all of whom pleaded guilty.

Apple Daily was forced to close in 2021 following police raids.

Lai did not submit a letter during sentencing arguments asking the court for leniency, according to local media.

His defence lawyer Robert Pang told the court a lengthy jail term would be "harsher" for someone of Lai's age and physical condition.

"Every day (Lai) spends in prison will bring him that much closer to the end of his life," Pang said at the time.

Prosecutors cited in response a prison medical report that said Lai's "general health condition remains stable", and that he had no complaints after being treated for problems with his heart, teeth and nails.

Lai was kept in solitary confinement at his own request to avoid harassment, prosecutors said.

Two of his children have raised concerns over his health in recent months, but authorities said Lai has received "adequate and comprehensive" care.

As of the start of the month, Hong Kong has arrested a total of 386 people for various national security crimes, with 176 of them being convicted.