Royal biographer Robert Lacey believes that the relationship between royal siblings Prince William and Prince Harry is critical to the future of the British royal family. The new book "Battle Of Brothers" gives an uncompromising account of the feud between sons of Prince Charles and late Princess Diana

The "Battle of Brothers: William, Harry and the Inside Story of a Family in Tumult," is being serialised in the Daily Mail on Saturday and next week. It presents a devastating picture of the fall-out between the brothers who shared a close relationship once. It traces their lives, and that of their very different spouses — from the moments they were born.

The book promises to take readers inside the historic Sandringham Summit, at which Queen Elizabeth II, Princes William, Harry and Charles determined the terms of the Sussexes' exit from their roles as working royals.

"The idea of our modern royalty was built on the idea of these two brothers. What you've got to realise is that the whole strategy of the monarchy was based on them sticking together. Meghan changed all that. She is difficult. She has an incredible and dangerous level of self-belief," said Robert Lacey, historical consultant of Netflix's "The Crown."

"But the Palace got this very wrong, as it always does with the second-born. They always treat the second-born badly, not to say cruelly. It happened with Princess Margaret. It happened with Prince Andrew. It's the classic heir and the spare thing. They just don't know what to do with the spare. And they certainly didn't know what to do with the spare's wife," the historian added.

Lacey spent months talking to royal insiders about when the cracks in the brothers' relationship started appearing, and how the system of monarchy conspired to fan the flames of resentment rather than broker a compromise between the two.

"When I started to look into this supposed feud between the two princes, I didn't believe it. I thought it was newspapers stirring up something that wasn't there. I didn't want to believe it, in truth. None of us does. Yet it most definitely exists. Actually, it's worse than anyone thinks," Lacey said.

The author added: "If this breach between the brothers is not healed in some way it will come to stand with the Abdication crisis and the death of Diana as one of the traumas that changed the monarchy."

Lacey told how he sent the draft of the book before publication to the Buckingham Palace, only for it to be returned to him with a stern cover letter. In short, the Palace reportedly didn't want to know the contents of the book.

Princes William and Harry reflect on their late mother Diana, Princess of Wales ITV

The book will hit the stands on Thursday, Oct.15.