Matthew Trickett, 37, was found dead in a park in Maidenhead, west of London, on Sunday following a report by a member of the public, Thames Valley Police said.

Trickett had more recently been employed as an immigration enforcement officer and private investigator, the UK's PA news agency reported.

"An investigation is ongoing into the death, which is currently being treated as unexplained," the police force said in a statement.

Trickett, from southeast England, was one of three men accused in the Hong Kong spy case.

He was formerly employed by the UK Border Force at Heathrow Airport, before joining Home Office Immigration Enforcement on February 21, 2024, PA said.

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

He was released on bail along with Chi Leung Wai, 38, and Chung Biu Yuen, 63, pending their next court appearance, scheduled to take place on Friday.

The three were 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.

Police said earlier the "foreign intelligence service" in question was Hong Kong's.

China's foreign affairs commissioner in its territory of Hong Kong "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".

- China spying charges -

The UK has been outspoken about the Hong Kong government's new national security law, which it views as eroding the territory's rights and freedoms.

Britain has repeatedly denounced the treatment of pro-democracy campaigners in its former colony, and launched a visa scheme to allow Hong Kong residents to come to the UK.

As a result, it has become a refuge for dissidents, including pro-democracy politician Nathan Law.

Hong Kong police last year issued a wanted list of eight overseas activists that included Law.

As part of the UK police investigation, 11 people were taken into custody this month.

The case involving Trickett comes 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.

Former prime minister David Cameron, now foreign minister, in 2015 hailed a "Golden Age" of ties between London and Beijing, but relations have deteriorated significantly in recent years.

As well as clashing over Hong Kong, the two countries have argued over the treatment of the Uighur Muslim minority in China's Xinjiang region and over human rights in Tibet.

Amnesty International said in a recent report that China was targeting citizens studying abroad for their political activism, with some students reporting harassment of family members back home.

Asked about the Amnesty report, Beijing's foreign ministry dismissed it as "purely malicious smears".