A new unofficial book about Prince Harry's decision to step down as a senior member of the British Royal Family is to be released later this month.

'Royals at War' promises to explore the circumstances that led to one of the most dramatic incidents in modern royal history, as the Duke and Duchess of Sussex decided to distance themselves from the monarchy.

A teaser for the book - which is released in e-book form on May 26 and hardback on June 2 - reads: ''Through revealing interviews with royal family insiders, friends, aides, historians, royal watchers and others with intimate knowledge of The House of Windsor, this tell-all book looks back at the events, motives and crises which led Harry (sixth in line to the throne) dramatically abandoning his birthright - in a move not seen for nearly a century, when Edward VIII also gave up the crown for the woman he loved as Europe teetered on the brink of fascism and war. Like Edward and Wallis Simpson, the catalyst for scandal here is also an ambitious, controversial American woman.''

The book is set to chart how the royal couple's relationship with the British public has soured after they tied the knot at St George's Chapel at Windsor Castle in 2018.

The teaser continued: ''As senior royals expressed disapproval, the public at first seem to enjoy the royal spat, with many still supporting Team Meghan - until it emerged that the bill to renovate Frogmore Cottage to Meghan's lavish expectations would be $3 million and would be picked up by British taxpayers.''

'Royals at War' - which is written by best-selling author Dylan Howard and entertainment journalist Andy Tillett - will ask why and how Meghan Markle riled up the British monarchy after marrying Prince Harry.