"My grandmother, who worked for the Queen, told me outright that I was Camilla and Charles' son many times," he told New Idea.
The engineer, born in England, says there are other strange details about the time he was born. The hospital where he was told he was born did not deliver babies at that time. Moreover, the parents' names on his birth certificate turned out to be fictitious.
He says he, not William, is second in line to the throne.
Dorante-Day has filed legal proceedings in Australia's High Court to prove he is Charles and Camilla's son and the case is currently being assessed, he says.
"This is most explosive thing that's ever happened to the palace," he said.
Spotted this image on the Spanish newspaper La Verdad. Interesting comparison!
Funny how they "say" oh, it's only...
Posted by Simon Charles Dorante-Day on Thursday, 30 May 2019
"It's definitely the most significant step I've taken so far – I've had to force a deadline, hold them to a date, because we need answers."
Court staff reportedly laughed it off when he first filed papers but the man is adamant his "40-year search" will lead him to the truth.
Dorante-Day says the royal family would have found out about the court case before Christmas and has no doubt it came up during Megxit negotiations.
"While the whole world was thinking they were talking about Harry, we believe this legal battle would've also been on the agenda and discussed," he said.
Clarence House told the Mirror they "did not have any comment to make on the story".
