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Fifty-five-year-old Simon Dorante-Day was born in April 1966 and adopted by a family in Portsmouth that were linked to Buckingham Palace.
He told 7 News: "My grandmother, who had worked for the Queen, told me outright that I was Camilla and Charles' son many times."
So convinced is Mr Dorante-Day that he has been to the Australian High Court to fight his case, having failed with three previous legal attempts to prove his claims, according to The Daily Mail.
According to his claims, he was conceived when Charles and Camilla were just 17 and 18, respectively.
But while neither Charles nor Camilla have ever responded to Mr Dorante-Day's claims, he continues to believe.
He has now released a new photograph of his son Liam, which he has compared with that of Queen Elizabeth II at a similar age, and he thinks this shows the link.
He said: "We were blown away when we first saw it.
"Elvie [his wife, Dr Elvianna Dorante-Day] and I feel like we see many members of the Royal family in our children, but this one was an eye opener.
"One of my supporters did the comparison and sent it to me and I was speechless.
"A lot of people are reacting to it because it's hard to deny the similarity."
Some people online seem to have been convinced by the pictures.
One person commented: "Yes, I saw the resemblance of Liam to Elizabeth in the photo you shared today and thought, wow! Too many similarities between yourself, your children and others for it to be a mere coincidence or conspiracy theory."
And this isn't the only piece of pictorial evidence that Mr Dorante-Day has shared.
The father has also shared photos of himself in comparison with images of Camilla and even the late Duke of Edinburgh, which again he believes show a clear similarity.
And while there will be many out there who aren't convinced by his story, he and his wife say they are committed to finding out the truth.
He added: "We owe it to our children for them to have answers.
"We have some exciting new legal paths that we're looking at to uncover the truth.
"At the end of the day, I'm just a man who's looking for his biological parents - and every road takes me to Charles and Camilla."
Featured Image Credit: PA
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