Prince William and Prince Harry's relationship has grown complicated over the years, with the latter himself publicly admitting about differences with his elder brother. A book about the two British princes due in October this year, "Battle of Brothers: William and Harry — the Friendship and the Feuds," will further elaborate on the much-publicised rifts between the royal siblings.

In a press release obtained by Us Weekly, royal historian Robert Lacey who researched the differences between Prince William and Prince Harry for the book said that he was "moved to tears" during the process. The book will also cover the parts the princes' wives, Duchesses Kate Middleton and Meghan Markle, played in their ups and downs.

The historian is also the official royal consultant to "The Crown" TV series which covers the reign of the duo's grandmother Queen Elizabeth II. He stated: "I have been astonished, elated and sometimes moved to tears by the fresh details and insights I have discovered in researching this story of family conflict."

Lacey added that it's been "both enthralling and painful to trace this drama" through conversations with people intimately involved with the royal family. He also hinted that the feud between the children of Prince Charles and Princess Diana was much more critical, but didn't make that big of an impact because of how the British monarch handled the crisis in her family.

"This dispute between brothers ranks alongside the abdication of King Edward VIII in its impact upon the British monarchy and how it operates, The Queen herself has handled the crisis superbly – you might not even guess there is a crisis at all from her calm and stoic manner!" Lacey stated.

"As for Princes William and Harry, the two main protagonists in the rift, they have also acted with honesty and decency within the system, as have their wives," he added.

Lacey also said that the problem between the royal siblings is partly due to their contrasting personalities, but their issues have been made worse by the way the British monarchy works. While William is second in the line of succession to the British throne and is expected to hold the position one day, Harry who is sixth in the line of succession has quit as a working royal of the British royal family along with his wife Meghan.

"These two brothers – once so close as to seem inseparable and now separated by an ocean and by much more than mere distance – have been acting out the profound contradictions that go back into their childhoods and even before that: into their parents' ill-fated" marriage and their father Prince Charles' "compulsive passion for Camilla," the release stated.

William and Harry at Pippa Middleton wedding
Prince William, Duke of Cambridge and Prince Harry arrive for the wedding ceremony of Pippa Middleton to James Matthews at St Mark's Church on 20 May Arthur Edwards - WPA Pool/Getty Images

"We have seen conflicts between heir and spare in every recent generation of the royal family – but nothing so profound as this," Lacey continued, adding that it is unclear if the duo can ever be as close as they once were.