The alleged disagreement between the royals is detailed in a new biography about the Princess of Wales

King Charles and Prince William were involved in a dispute earlier this year surrounding the safety of the royal family flying in a helicopter, a new book claims.

According to Robert Jobson’s soon-to-be-released biography, Catherine, The Princess of Wales, the King, 75, “raised concerns” with William, 42, over his helicopter use with Kate Middleton and their children, Prince George, Princess Charlotte and Prince Louis, that sparked a tense disagreement between the pair.

In an excerpt from Jobson’s book, per The Daily Mail, King Charles brought up his worries on the matter after coming to terms with his mortality following his cancer diagnosis, which was announced in February.

The King even presented experienced pilot William with “a formal document acknowledging the risks involved and taking full responsibility for his actions” amid their dispute, the new book claims.

Charles’ concerns echoed that of his late mother Queen Elizabeth, who previously requested William not fly with his family on a helicopter from Kensington Palace to his former residence of Anmer Hall in Norfolk, which is a 115-mile journey, according to Robson.

Queen Elizabeth “didn’t hold back” on her opinion, an aide shares in the new book, after being haunted by the 1967 helicopter crash that 𝓀𝒾𝓁𝓁ed her flight captain.

PEOPLE understands that Buckingham Palace does not comment on private conversations or His Majesty’s views, but they do take into account the safety and security of others when deciding on the most appropriate means of travel for the royal family.

It is also understood that the palace has fully evaluated the risk of the family flying together, compared with implementing a policy where separate travel assets are provided for one or more members of The Prince and Princess of Wales’ family.

Elsewhere in the book, Kate, 42, who is currently receiving cancer treatment, is praised as having a “stabilizing influence” on William — particularly when it comes to tensions between the prince and his father.

A source told Jobson that Kate “is somebody who always tries to see both sides of any dispute” and is the “emotionally mature” one in their relationship.

King Charles also values Kate’s positive effect on his eldest son, according to the book. “I think the [King] understands Catherine is a good influence on the entire family. He loves and truly appreciates everything she does,” a courtier said.

Catherine, The Princess of Wales will be available to purchase wherever books are sold on Aug. 6.