Donald Trump’s success in avoiding the Vietnam War draft nearly fifty years ago is emerging as a new trouble spot for his see-saw campaign as he battles to contain the fall-out from his feud with the parents of a Muslim-American soldier slain in Iraq.

He has never denied that he benefited from five draft deferments that saved him from the conflagration in Southeast Asia, four to allow him to continue his studies and one thanks to a doctor’s note saying he had a spur on a heel bone. But new questions and possible inconsistencies have surfaced, particularly with regard to the one medical deferment.

In a 2011 interview with a Fox News channel in New York, Mr Trump spoke of the “amazing” moment as a college student when he was given a “ a very, very high number” in the Vietnam draft lottery, which, he said, effectively ruled out his being sent to serve.

But, as a New York Times article now highlights, that lottery actually happened long after Mr Trump had finished his final round of studies at the Wharton School of Finance. Moreover, the medical classification he had received because of the bone spur, known as a 1-Y, meant he was disqualified anyway, rendering the number he received in the lottery draw - high or low - irrelevant.

“For all practical purposes, once you got the 1-Y, you were free and clear of vulnerability for the draft, even in the case of the lottery,” Richard Flahavan, a spokesman for the Selective Service System, the US military conscription agency, told the paper.

Mr Trump is likely meanwhile to come under renewed pressure to explain how it was that a successful athlete at college - and as a 70-year-old today with an apparently unblemished health record - he found himself diagnosed with a heel spur serious enough to stop him serving.

“I had a doctor that gave me a letter - a very strong letter on the heels,” Mr. Trump said in an interview with The Times last month. He added that “over a period of time, it healed up”. When questioned to say which heel was affected by the spur, Mr Trump avers that he can’t recall.

Donald Trump speaks about Vietnam

Past presidential candidates have faced awkward moments when asked to explain how they avoided service in Vietnam. They included George W. Bush and Bill Clinton, both of whom nonetheless went on to win election to the White House.

On Tuesday, one of the nominee's sons, Eric Trump, attempted to suggest that the entanglement with the Khan family was somehow steam in a tea cup. "I think this is something that was honestly blown hugely out of proportion,“ he told CBS News.

But the timing for Mr Trump is especially bad because of his feud with Khizr Khan, the father of the fallen soldier, who provoked his fury by taking the stage last week at the Democratic convention and asserting that the nominee "had sacrificed nothing and no one,” in his life.

It surely also doesn’t help that in a 1997 radio interview with shock-jock Howard Stern, somehow Mr Trump managed to suggest that avoiding getting sexually transmitted diseases had been his “personal Vietnam”.

“I’ve been so lucky in terms of that whole world. It is a dangerous world out there. It’s scary, like Vietnam. Sort of like the Vietnam-era,” Mr Trump is seen to say a video that was recently found by the Buzzed website. “It is my personal Vietnam. I feel like a great and very brave soldier.”

The attacks launched by Mr Trump against Mr Khan and his wife, including the assertion that Ms Khan was silent on the convention stage because she had not been given permission by her husband to speak, have prompted widespread outrage, including from within the Republican Party.

On Monday, Senator John McCain vigorously repudiated Mr Trump’s remarks. A Vietnam prisoner of war, Mr McCain himself suffered ridicule from Mr Trump during the primaries when the latter suggested it was wrong to call him a hero, because he shouldn’t have allowed himself to be captured in the first place. “I like people who weren’t captured," Mr Trump offered at the time.