LONDON: Britain called for Hong Kong to release Jimmy Lai ahead of the trial of the leading China critic and media tycoon on charges he colluded with foreign forces, including the United States.

David Cameron, who became foreign minister in November, toughened Britain's stance on Sunday by explicitly calling for the release of British national Lai, who faces possible life imprisonment.

Lai's long-awaited national security trial opens in Hong Kong on Monday, with foreign envoys and others saying it is a key test for the city's judicial independence and freedoms under the sweeping national security law imposed by China in 2020.

Lai, 76, the founder of now-shuttered pro-democracy newspaper Apple Daily and one of the most prominent Hong Kong critics of China's Communist Party leadership, has pleaded not guilty to all charges he faces in the new trial.

His son Sebastien Lai, who has met with Cameron to discuss the case, previously criticised Britain for being "incredibly weak" in standing up for his father.

"As a prominent and outspoken journalist and publisher, Jimmy Lai has been targeted in a clear attempt to stop the peaceful exercise of his rights to freedom of expression and association," Cameron said in a statement.

"I call on the Hong Kong authorities to end their prosecution and release Jimmy Lai."

Cameron said Hong Kong's National Security Law was a "clear breach" of the Sino-British Joint Declaration, which returned Hong Kong from British to Chinese rule more than 25 years ago under a deal guaranteeing a high degree of autonomy, including freedom of speech, under a "one country, two systems" formula.

"(The national security law) has damaged Hong Kong, with rights and freedoms significantly eroded," Cameron said.

"I urge the Chinese authorities to repeal the National Security Law and end the prosecution of all individuals charged under it," he added.

China has in the past responded to similar criticism by accusing Britain of acting with a colonial mindset.