According to a new book, Prince Harry was so incensed when his older brother, Prince William, questioned his fast-moving romance with Meghan Markle that he blurted out, “Who the hell do you think you are?”

The alleged outburst in September 2017 happened when Harry indicated that he was about to propose to his American girlfriend, prompting William to ask, “Why rush things?”

This is according to a palace insider who contributed to the new book by Christopher Andersen titled “Brothers And Wives: Inside The Private Lives of William, Kate, Harry And Meghan” (Gallery), out Nov.30.

The author dissects the one-time close bond between the so-called “heir and spare” and the ongoing drama that allegedly caused it to break.

The friction allegedly began when members of Harry’s inner circle became concerned about his whirlwind romance with the divorcee whom he met on a “blind date” in July 2016.

One of the skeptics was his uncle, Earl Spencer — the brother of the late Princess Diana — whom William drafted to underscore his opinion that the lovebirds should slow down.

The tactic, Andersen writes, made “matters far worse” because Harry was “furious that his brother was actively seeking to have others interfere in his personal affairs.”

Harry’s sister-in-law, Kate (above), and brother, William, reportedly wanted an “English rose” for him.

Harry soon become “angry” with anyone he perceived to be “against Meghan.”

Harry extolled his girlfriend’s virtues to his brother before she and William were introduced. But he had previously worried what William — who, Andersen writes, can be “very stiff” — would make of Meghan, fearing he’d think she was “an opportunist.”

William, the book claims, was initially charmed, but soon harbored doubts.

Andersen writes that William and wife Kate “wanted a proper English rose” for Harry, and became increasingly perturbed by headlines about Meghan’s father, Thomas Markle.

The Duke and Duchess of Cambridge, the book claims, were particularly “alarmed” by the caustic public warnings of Meghan’s estranged half-siblings, Samantha and Thomas Jr., that Meghan would be a poisonous addition to the Windsor family.

According to the book, William said of Harry: “It feels like I have lost my best friend.”

A spokesperson for William and Kate had no comment on the book. A spokesperson for Harry and Meghan did not respond to requests for comment.

Andersen writes that the escalating Markle drama went on to “mortify” Harry and Meghan and “horrify” William. Thomas Jr. was particularly adept at bashing his half-sister, telling reporters, “She thinks she’s another Princess Diana.”

“I don’t know what to believe,” the book claims William told a confidant.

He allegedly expressed his misgivings with Prince Charles, informing his father that Harry’s obsession with Meghan was “like something I’d never seen.” William went on to say that “it feels like I have lost my best friend.”

Undeterred, Harry and Meghan tied the knot on May 19, 2018, in Windsor, England. Now living in California, they have two children, Archie, 2, and 5-month-old Lilibet.