Simon Dorante-Day, who was born in the United Kingdom but now resides in Queensland, believes he was conceived in 1965, when Charles and Camilla were initially romantically involved.

The Australian grandpa has released fresh photographs as part of his decades-long quest to establish he is Prince Charles and Camilla, Duchess of Cornwall’s undisclosed child.

The engineer was born in Portsmouth in April 1966 and was subsequently adopted by a local family with royal connections.

He posted a picture comparing his son Liam to the Monarch when she was in her twenties, saying he was “blown away” when he saw the photos side by side for the first time.

Credit: Facebook

“I saw the resemblance of Liam to Elizabeth in the photo you shared today and thought, wow!” one Facebook user remarked on Mr Dorante-recent Day’s post sharing the photograph.

“Too many similarities between yourself, your children and others for it to be a mere coincidence or conspiracy theory.”

“Yes, I saw the resemblance of Liam to Elizabeth in the photo you shared today and thought, wow,” another said.

The strange allegations were made by the 55-year-old after Charles and Camilla did not reply to his letters.

“My grandmother, who had worked for the Queen, told me outright that I was Camilla and Charles’ son many times,” Dorante-Day told 7News.

Simon Dorante-Day claims the Duke and Duchess of Cornwall are his birth parents (Image: Simon Dorante-Day/Facebook)

“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.”

The pair would have been teenagers at the time, years before Charles’ disastrous marriage to Princess Diana and decades before they could finally be together openly.

Simon, who was born in Gosport, Hampshire, in April 1966, has even filed documents in the High Court requesting that the Duke and Duchess of Cornwall consent to a DNA test.

According to The Daily Mail, he is so sure that he has gone to Australia’s High Court to defend his case after three prior legal efforts to establish his allegations failed.

Prince Charles and Camilla have yet to react to the allegations.

(Image: Getty Images)

He said in 2019 that he thinks his truth would never have been exposed if Princess Diana had not died in 1997.

“We believe that Diana knew of my existence and that she had put the pieces together,” he said.

“I think Diana was at a point where she was finding out answers about her life, how she was wronged, and she was going to go public with it.”

He says that another recent royal picture has confirmed his belief that he is of royal lineage.

Since his death, images of the late Prince Philip as a younger man have been widely circulated, and Mr Dorante-Day claims that people have been “stopping me in the street to say how much I look like him”

Charles and Camilla tied the knot in 2005.