Had it been true, it would have been one of the most gripping war stories of all time.

But sadly it looks as if the man found living in the Vietnam jungle, who a new documentary claims is ‘long dead’ US army veteran Sgt John Hartley Robertson, is likely to be a fraud.

News of the “discovery” of Sgt Robertson swept the world yesterday, after details emerged of a soon-to-be-released documentary that claims an elderly man living in the remote Vietnam jungle is in fact a former Green Beret shot down and presumed dead 44 years ago.

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Although DNA tests had not taken place, tearful “reunions” with a former colleague and the last surviving sister of Sgt Robertson appeared to confirm the man was who he claimed to be.

80-year-old Jean Robertson Holly even went as far as saying: “There’s no question. I was certain it was him in the video, but when I held his head in my hands and looked in his eyes, there was no question that was my brother”.

That emotional story was shattered today, however, when it was claimed that the found man was in fact a fraudster who the US government performed DNA tests on 20 years ago and whose story had been fully debunked as an attempt to exploit Vietnam’s Missing in Action and Prisoner of War groups and claim military back-pay.

According to a memo sent to a UK news organisation yesterday evening, the man claiming to be Sgt Robertson is in fact Dang Tan Ngoc – a 76-year-old Vietnamese citizen of French origin who has a history of pretending to be US army veterans.

The memo, taken from a Defense Prisoner of War Missing Personnel Office report in 2009, apparently says Ngoc first came to the attention of the US military in 2006 when he started telling people he was Sgt John Hartley Robertson.

He was apparently questioned about the claims but quickly admitted he had been lying and was in fact Vietnamese.

In 2008 Ngoc apparently began claiming to be Sgt Robertson once again, and he was taken to a US embassy in Cambodia to be fingerprinted. It was quickly established that the fingerprints did not match those of the missing army veteran.

In the documentary, titled ‘Unclaimed’ and made by the Emmy-award winning filmmaker Michael Jorgensen, the man claims that he is no longer able to speak English after living in the remote Vietnam jungle for the last 44 years.

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The real Sgt Robertson is thought to have died when his helicopter was shot down during a special operation in Vietnam in 1968. His name has been etched on Vietnam War memorials and army records list him as having been “killed in action”.

The man went on to tell a gripping story of being kept in a bamboo cage and tortured for around a year by the North Vietnamese, before going on to marry the nurse who took care of him and brought him back to health.

He also said that decades old head injuries and the early stages of dementia meant he could no longer remember the names of his wife and children.

Although it now appears that all of this was a lie, it was still enough to not only convince the documentary makers that he was telling the truth, but also to win the full support of people who knew the real Sgt Robertson.

Reports suggest Ngoc could have been impersonating Sgt Robertson since around 1982, with some Vietnam War veterans saying he could have possibly conned veteran groups out of thousands of pounds over the last 30 years.

In 1991 Ngoc attracted the attention of former CIA Paramilitary Operations Officer Billy Waugh, who was involved in the capture of Carlos the Jackal and who later tracked Osama bin Laden through the Tora Bora Mountains in the wake of the September 11 attacks.

Waugh led a team of investigators into the Vietnamese jungle and was able to take DNA from Ngoc.

After Waugh’s visit, Ngoc’s name became synonymous with conmen impersonating US army veterans that are missing in action. There is still a huge amount of anger among legitimate Vietnam veterans at the deception.

The Unclaimed documentary came about after Vietnam veteran Tom Faunce heard about an “army brother” who’d been shot down 40-years earlier, listed as deceased in action and “forgotten about by the US government”.

Determined to make good on his army vow never to leave a man behind, Faunce teamed up with Jorgenson to track the mystery man down and find definitive evidence that either proved he was Sgt Robertson, or outed him as a fraud.

Jorgenson says he was sceptical about the project from the off, not least because of Faunce’s stated intention to reunite a man who had been missing and presumed dead for 44 years with his now fully recuperated family.

They agreed that one intention of the film would be to try and get the man they believed was claiming to be Sgt Robertson to admit he’d been making the whole thing up.

“The MIA story was pretty unbelievable, pretty grandiose”, Jorgenson told the Globe and Mail, adding “Tom went to meet him and was very sceptical, grilling this guy up and down, trying to get him to break”.

But as the man told the pair his story via a translator; how he’d been captured by the North Vietnamese shortly after his helicopter was shot down, and how he’d been held in a bamboo cage and tortured for around a year, the two men became convinced he was Sgt Robertson.

That confidence would now appear to have been misplaced.