In a series of never-before-seen photos taken while the princess-to-be vacationed with Prince William in Ibiza, Spain, in 2006, Middleton shows off her tanned and toned physique in a sexy white bikini.
Since she began dating the future king eight years ago, Middleton has diligently been preparing for her royal close-up.
The 28-year-old -- who quit her job at her family's party supply business to prep full time for her wedding -- has transformed from "a university student into a woman," Simone Simmons, a close pal of the late Princess Diana, tells Us. "I call it that she lost her puppy fat!"
How does Middleton maintain her hot bod? With regular workouts at London's Clarence House gym and beauty treatments at Richard Ward Hair and Metrospa in the British city's Sloane Square.
"Kate is always perfectly manicured, and she will have blow-dries before she goes out," says Katie Nicholl, royal watcher and author of William and Harry: Behind the Palace Walls.
Still, commoner Middleton sometimes finds herself overwhelmed by all the fuss. "Now, she has a full entourage in tow," Nicholl tells Us. "The enormity of what she's in is finally sinking in."
For more on the Royal Wedding, including rare photos, interviews with family and friends, details on Kate's new life as a princess and a tour of their lavish homes, order a copy of Us Weekly's Will & Kate: The Royal Weddingnow.