UK authorities will not investigate the circumstances surrounding the death of an British conspiracy theorist in Poland, the Foreign Office has revealed.

Two days before he died, Max Spiers allegedly told his mother: "Your boy's in trouble. If anything happens to me, investigate."

The father-of-two, from Canterbury, was found dead on a sofa in a Warsaw apartment on 16 July.

Polish authorities said Mr Spiers died from natural causes, but his mother claims no post-mortem examination was carried out on his body in the country, which he had visited to give a series of speeches and interviews about his investigations.

The 39-year-old was prominent in conspiracy theorist circles, and had made a career out of investigating UFOs and alleged cover-ups. His mother, Vanessa Bates, 63, said he had recently moved on to investigating politicians, business people, and celebrities.

Friends of Mr Spiers said he died after vomiting a black liquid and complaining of migraines for several days. They believe the circumstances surrounding his death were suspicious.

But the Foreign Commonwealth Office (FCO) said it was not investigating the death as it happened abroad.

A spokesperson told The Independent the FCO provided assistance to the family of a British national following his death, but explained the UK had no jurisdiction to investigate the death of British nationals overseas.

They said investigations into Mr Spiers' death were the responsibility of the Polish government.

It comes after Ms Bates gave an interview in which she said her son’s investigations into UFOs and government cover-ups may have made him enemies who wanted him dead.

She said he was “exposing” who was “dangerous in the world of government [and] the entertainment world".

“These people seemed to be involved in some very, very dark and dangerous areas of the world and I was afraid that as he was gaining popularity and fame that perhaps somebody would want him out of the way i.e. not alive any longer," she said in the interview posted on Youtube.

Ms Bates described her son as “incredibly strong, incredibly fit, nothing wrong with him at all".

She said he was "in incredibly good spirits” before he left Kent, where he had been staying with her, and suggested he was unlikely to have died of natural causes.

In Warsaw, Ms Bates said: “He was staying with a woman who he had not known for long and she told me how she found him dead on the sofa.

“But I think Max had been digging in some dark places and I fear that somebody wanted him dead.”

Ms Bates said her son had suffered from migraines in the days before he died and was taking antibiotics when she last spoke to him on 14 July. He also complained on social media of being “way too tired” around this time, and cancelled an interview because of his health.

He was found dead on 16 July.

In the last interview he gave, Mr Spiers discussed the Fourth Reich with a Polish youtube channel. He claimed Nazis and Zionists were “working in alignment” after a war stretching back “50,000 years”.

He said: “If you look at the word Zion you just change a vowel around and switch the letters around and you have the word Nazi.

“I don’t believe they were always running with each other but I do believe they were at odds. This war stems back 50,000 years. They just weren’t called Nazi and Zion, that’s a modern-day nom.”

Mr Spiers also spoke about Dulce, which he believed was a secret underground alien base in New Mexico.

Mr Spiers said he had seen inside a base where he claimed children were being used for their “pure energy”.

He said: “I have seen a vat of children in disassociated states, some crying, some not and some completely gone.

“They also have female breeders down there constantly breeding children, thousands of them.“

Although his theories appear farfetched, many of Mr Spiers online followers believe he may have been murdered because of his investigations.

On one website, Project Camelot, a blogger wrote: “The entire circumstances are suspicious and I urge everyone to encourage the release of details about what really happened and call for an autopsy.”

Another, Craig Hewlett, added: “If it wasn't true what he talks about then why would they kill him? Healthy people don't just get sick and die, they get poisoned.”

Miles Johnson, who runs supernatural conspiracy theory group the Bases project and knew Mr Spiers personally, said in an interview with Project Camelot: “A person has died here and I don't think it's good enough [that] somebody who just took normal medication should end up vomiting, spewing black liquid, whatever it was. And then shortly after that, whatever length of time it was, he died.”

Ms Bates said: “All I have is a death certificate from the Polish authorities that it was from natural causes, but no post-mortem was done, so how can they tell that?

“They are also refusing to release any paperwork about it to me because, absurdly, I don't have his written permission.”

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Mr Spiers was buried in Canterbury cemetery after his mother arranged to have his body flown home a week after his death.

Because Mr Spiers' body was repatriated, a UK coroner's investigation could be initiated and a post-mortem examination was carried out by a pathologist in Kent. However, Ms Bates said she still did not know the result more than two months later, or whether there would be an inquest.