Meghan Markle pulled out of Pippa Middleton’s high-profile wedding at the last minute — because she knew it would turn into a spectacle over who had the best butt, according to her new biography.

Pippa — whose derrière stole the show at her sister Kate’s 2011 royal wedding to Prince William — was worried her own nuptials would be “overshadowed” by the “media frenzy” surrounding Markle and then-golden-boy royal Prince Harry, the authors claim.

It made the “Suits” star a “somewhat reluctant addition to the guest list” for Middleton’s May 2017 wedding to James Matthews, according to the new bio, “Finding Freedom: Harry and Meghan and the Making of a Modern Royal Family.”

Markle “put careful thought into her outfit” to make sure it was “stylish without being splashy,” the book says.

But with just hours to go, she decided not to attend when The Sun newspaper ran a cover story with a headline suggesting it would be “Meghan v Pippa” in the “Wedding of the Rears,” the bio says.

“Harry and Meghan agreed that she couldn’t possibly turn up at the church” after the story suggested Markle was about to upstage Middleton with her own “Pippa moment.”

“If they worried their arrival might create a media circus despite taking every precaution, now they had no doubt it would,” the book states.

Instead, the actress checked into a nearby Airbnb while her prince went to the church service alone — later sneaking into the reception in a “decidedly unflashy long black gown” and having done her own makeup, the book says.

Ironically, Markle’s then-best friend, Jessica Mulroney, ended up getting the headlines — with a rear shot of her in a figure-hugging blue gown catching attention in the way the bride had at her sister’s wedding.

The Duke and Duchess of Sussex have insisted that they “were not interviewed and did not contribute” to “Finding Freedom.”

However, the authors’ note says that “when appropriate” they spoke to “the couple themselves.”