Just days before his death, the dad-of-two had texted his mum Vanessa Bates the chilling message: "Your boy's in trouble. If anything happens, investigate."

Mr Spiers, a world-renowned conspiracy theorist from Canterbury, Kent, was said to have been probing into the lives of well-known figures in politics, business and entertainment.

He made a name for himself investigating UFO sightings and alleged cover-ups.

His mum, 63, said: "I think Max had been digging in some dark places and somebody wanted him dead."

Mr Spiers, who went to school with Orlando Bloom, had visited Poland after holidaying in Cyprus with a friend, who later discovered his body.

The inquest at Canterbury coroner's court into his death in Otwock on July 16 opened on Monday after his body was repatriated.

Polish authorities had initially dismissed his death as being down to natural causes, but a post-mortem examination at Margate Hospital, Kent, was unable to determine how he died.

In a statement, coroner's investigations officer Caroline O'Donnell said: "When they returned, Mr Spiers became ill with a high temperature and was weak.

"The following day, Mr Spiers vomited two litres of black fluid.