The explosive book is not due to launch until January 10, but it was reported on Thursday that some copies have accidentally gone on sale five days ahead of time.
The Spanish version of the memoir is entitled En La Sombra, which translates as “In the Shadow”.
A copy purchased in Spain on Thursday shows it is dedicated “Para Meg, Archie y Lili... y, for supuesto, mi madre” - “For Meg, Archie and Lili... and, of course, my mother”.
Images published by The Sun show the book open on a page bearing a black-and-white photo of the Duke as a young boy with his mother, Princess Diana, alongside the words “Primera parte - Desde la noche que me envuelve”, or “Part one - Since the night that surrounds me”.
Although not officially due to hit the shelves until Tuesday, Spare made headlines around the world on Thursday after it was apparently leaked to the press, and a number of extraordinary claims reportedly made in the volume came to light.
Prince Harry, then 20, caused outrage when he was pictured wearing the soldier’s uniform complete with a swastika armband for a fancy dress party.
But he alleges in Spare that it was Will and Kate who said he should go dressed as a Nazi, when he was choosing between that and dressing as a pilot.
He reportedly goes on to say they “howled with laughter” at the sight of him wearing it.
In another segment, the Duke reportedly claims he was physically attacked and knocked to the floor by his brother Prince William during a row over his marriage to Meghan.
According to the Guardian, which has seen a copy of the book, the altercation culminated with William grabbing Harry by the collar and throwing him to the floor, shattering a dog bowl with his back.
A spokesperson for the Spanish publisher, Barcelona-based Plaza y Janes Editores - which belongs to multinational conglomerate Penguin Random House - said: “A very clear launch protocol was established and communicated to all customers so that the book would not be marketed before that date.
“Everything points to the fact that some customers have breached their commitment to the publisher and have put the book on sale before the agreed date.”
As the allegations caused shockwaves on Thursday, a brand new trailer dropped for Prince Harry’s blockbuster interview which is due to air on ITV this Sunday, two days before Spare goes on sale.
In the one-minute clip, the Duke of Sussex reveals to ITV News at Ten presenter Tom Bradby that he still believes in the monarchy, though doesn’t know whether he will play a part in its future. He also fails to confirm whether he will attend his father’s coronation, which will be in May.
Instead, he says: “There’s a lot that can happen between now and then... But, the door is always open. The ball is in their court. There’s a lot to be discussed and I really hope that they’re willing to sit down and talk about it.”