The third defendant -- Matthew Trickett, 37, an immigration enforcement officer and private investigator who had previously been a member of the Royal Marines -- was found dead in a park in Maidenhead, west of London, on Sunday.

At a hearing at London's Old Bailey court, prosecutor Alistair Richardson said the Crown Prosecution Service (CPS) notified the court that the case against Trickett would be formally "discontinued".

"As has widely been reported, Matthew Trickett died on May 19 this year. The cause of his death is currently given as unexplained," Richardson said.

Thames Valley Police said later that following a post-mortem examination completed by the Home Office on Wednesday, they were not treating his death as suspicious.

His co-defendants in the case Chi, (Peter) Leung Wai, 38, and Chung Biu Yuen, 63, appeared at the hearing but spoke only to confirm their names.

Judge Jeremy Baker adjourned their cases until October 25 and provisionally fixed a five-week trial to start on February 10, 2025, at Kingston Crown Court in southwest London.

Trickett had previously been employed by the UK Border Force at Heathrow Airport, before joining Home Office Immigration Enforcement on February 21, 2024.

He was also the director of MTR Consultancy, a security firm formed in April 2021.

All three defendants had been charged with assisting a foreign intelligence service and with foreign interference, in violation of the 2023 National Security Act.

The act came into force in December and is designed to bolster UK national security against "hostile activity" targeting the country's democratic institutions, economy and values.

China's foreign affairs commissioner in its territory of Hong Kong has "strongly condemned" Britain for "cooking up charges" and accused it of a "vicious intention to interfere" in Hong Kong's affairs.

The office warned that Britain would receive "China's firm and strong retaliation".

The allegations came after two men, one of whom works in the UK parliament, were last month charged with spying for China. They are due to be tried next year.

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