A prominent British conspiracy theorist who died suddenly after vomiting a dark fluid had “potentially fatal” levels of an opioid in his system, an inquest has heard.

Max Spiers, from Canterbury, had travelled to Poland in April 2016 to speak at a conference about his research into extraterrestrials and alleged government cover-ups, The Guardian reports. He died on 16 July, at the age of 39, at the Warsaw home of a woman with whom he had begun an “informal relationship” during his trip.

Polish authorities initially concluded that Spiers had died of natural causes, but his friends, family and fellow conspiracy theorists rejected this claim and called for a full inquiry.

“I think Max had been digging in some dark places and I fear that somebody wanted him dead,” Spiers’ mother, Vanessa Bates, told The Daily Telegraph in October 2016.

Bates claims that just two days before he died, her son told her: “Your boy’s in trouble. If anything happens to me, investigate.”

In a more recent interview, she said Spiers’ efforts to look into UFOs and supposed government cover-ups had made him a target, The Independent reports. She also suggested that her son may have been researching an organisation he believed was involved in running a paedophile ring.

He was “incredibly strong, incredibly fit, nothing wrong with him at all” and in “incredibly good spirits” when he left the UK to go to Poland, she added.

Bates claims that when his belongings were returned to her, his laptop had been wiped of all data and the sim card was missing from his phone.

However, despite her suspicions, an inquest yesterday heard that the cause of Spiers’ death was “pneumonia and intoxication by drugs”. A pathologist found that Spiers had “potentially fatal” levels of oxycodone, an opioid drug, as well as twice the therapeutic dose of a Xanax equivalent in his system.

The inquest, in Sandwich, Kent, heard that he had previously been addicted to heroin and crack cocaine.

The woman with whom Spiers had been staying in the Polish capital, Monika Duvall, said he had bought eight to ten boxes of the Turkish equivalent of anti-anxiety drug Xanax while the pair were on holiday in Cyprus, where it was available without a prescription.

He also occasionally “felt weak and sometimes he had problems with focus and attention” and had taken a number of the tablets before falling asleep on her sofa on the evening he died, Duvall said.

She claims that several hours later she noticed he had stopped breathing and that he began to vomit a dark black liquid when she tried to resuscitate him.

“I noticed he had something in his mouth, some remnants of food, so I turned him on to one side and saw gastric fluids pouring out of him – brown liquid, like somewhat tea coloured,” she said.

Spiers was declared dead at the scene and the Polish prosecutor’s office promptly ruled out “participation of further persons”.

The inquest continues.