A never-before-seen Mario Testino picture of Britain’s Prince William and bride Catherine Middleton will be the Vanity Fair July cover — 14 years after William’s mother, Princess Diana, appeared in the magazine in a now-famous series of portraits by the top fashion shooter.

Diana’s favorite photographer — whose iconic images of her appeared in the July 1997 issue — was so trusted that William and Kate chose him to shoot their engagement last year, and another Testino picture appeared in their official wedding program.

But he saved an engagement image that will be the cover of VF’s July issue, out next week ahead of the couple’s July visit to LA. The spread will also include shots of Diana, Prince Charles and Prince Harry.

PHOTOS: PRINCE WILLIAM AND KATE MIDDLETON

Testino, 56, who was honored Thursday at the Museo del Barrio gala, told us how he got William and Kate to relax during the shoot. He said, “I perform for people, I make them laugh, I make a fool of myself. I jumped around.” But he was honored to be chosen to do their pictures, saying, “I guess it was a way for them to keep William’s mother in the whole process of it.”

Asked to name his favorite career subject, he said, “It would have to be Princess Diana.” He added, “I am interested in working with down-to-earth, real people. I bond with all the real girls, Kate Winslet, Gwyneth Paltrow, Demi Moore, Kate Moss.”

Oscar-winning Winslet — who joked, “The two other Kates, Kate Moss and Kate Middleton, were unavailable, so you have to make do with me” — presented Testino with an Asprey silver tray signed by friends including Tom Ford, Julianne Moore, Julia Roberts and Charlize Theron.

Guests at the gala, which raised $1 million for the city’s top Latino cultural institution, included Anna Wintour, Grace Coddington, former French Vogue editor Carine Roitfeld, Donatella Versace, Courtney Love, Josh Hartnett, Candice Swanepoel, chairs Yaz and Valentín Hernández, Alex González and Raúl Martínez.