New book claims spending caused split between Prince William and Prince Harry

The latest rumour to surround the royal rift between brothers Prince Harry and Prince William puts the issue down to money.

In their new book Royals at War, released this week, authors Dylan Howard and Andy Tillett claim a series of extravagant splurges made by the Duke of Sussex put a strain on the pair's relationship.

"Harry's [spending] transformation is revealed as one of the fundamental factors behind the deep fissure that opened between him and his brother, Prince William," the book states.

As the Daily Mail reports the book also reveals Meghan's spending "rang alarm bells" with the "traditionally conservative" Queen Elizabeth.

"Growing up in the war left the monarch with a built-in sense of frugality and economy, despite being one of the richest women in the world," it claims.

Howard, an Australian tabloid journalist, and Tillett, a UK journalist who previously wrote a biography about Princess Diana, claim the excessive spending began after Harry decided to move out of Kensington Palace, where he had lived near the Duke and Duchess of Cambridge.

The book also call Harry and Meghan's wedding into question, spending upwards of $10,000,000 ($14,482,500 AUD) more than the estimated cost of Prince William and Kate Middleton's 2011 nuptials.

In addition, £2,400,000 ($AUD4,305,000) was spent on refurbishing Frogmore Cottage prior to the Sussexes moving in - money the couple have since begun to repay following their decision to step down as senior royals.

Currently residing in Tyler Perry's mansion in Hollywood, the pair are reportedly paying $US22,000 ($AUD32,000) a month as part of their plans to refund the $US3,000,000 ($AUD4,344,800) in taxpayer funds used for the initial refurbishments to their Windsor property.

The also book alleges Kate Middleton cautioned Harry about marrying Meghan, writing: "She gently reminded him that he was dating someone with a completely different life, past, and career and it would take time, care and attention for them to integrate."

Royals at War claims to be an "explosive expose" that "takes readers inside a riven Buckingham Palace to provide the definitive account of the unfolding abdication crisis of 2020," a description on Amazon reads.

Claiming to provide "revealing interviews with royal family insiders, friends, aides, historians, royal watchers, and others with intimate knowledge of The House of Windsor", the "tell-all book" promises to explore "the events, motives and crises" that led to Prince Harry and Meghan Markle stepping down from their senior duties.

The Sussexes and Cambridges have neither collaborated with the authors nor commented on the book's release.