British police are investigating the unexplained death of a man, who was charged earlier this month for allegedly helping Hong Kong authorities gather intelligence in the United Kingdom.

Matthew Trickett was due to appear in court on Friday to face charges under Britain's National Security Act.

British police said Trickett, who was 37, was found dead in a park in Maidenhead, west of London, on Sunday afternoon after a report from a member of the public.

Police said an investigation was ongoing into the death and an autopsy would be conducted.

Trickett’s lawyer said he was "shocked" by the news and was supporting Trickett's family.

The suspect was a former Royal Marine who'd worked for the UK border force. He was also the director of a security consultancy.

In early May, Trickett was arrested along with Peter Wai and Bill Yuen, the office manager of Hong Kong's Trade Office in London.

The three men were granted bail and were due to appear in court on Friday.

When they were charged, the Chinese Embassy in the UK said it made serious representations to UK officials over the matter, saying it firmly rejected and strongly condemned "Britain's fabrication of the so-called case".

The SAR Government, meanwhile, said it had "demanded the UK Government provide full details of relevant information of the alleged matter. (Agencies)