Testino's reps confirmed to the Telegraph that he was commissioned to take the portraits of the future king and his bride-to-be last month, shortly after they announced their intention to wed.
The sure-to-be-stunning portraits are expected to become public later this month, the newspaper reported.
"Mario always strikes up a fantastic rapport with his sitters, so above all the photograph is likely to reflect the happiness, love and laughter that the couple clearly share," Hilary Alexander, The Telegraph's fashion director, said.
The choice of the Peruvian lensman, best known for capturing mesmerising images of celebrities such as Gwyneth Paltrow, Madonna and Kate Moss, is another nod by Prince William to his late mother, whom he has said he wants to "keep close" to his happiness. His first gesture to her legacy was giving his fiancée his mother's diamond-and-sapphire engagement ring.
For of all of the famous clients in his portfolio, Testino's images of Princess Diana are considered his most iconic.
The princess sat for Testino in 1997 to promote a charity auction of her dresses at Christie's. The images were first published in Vanity Fair in July 1997, just a month before she died in a Paris car crash. The photos - which showed her lounging on sofas, her hair pushed back in a sleek new 'do - were also the subject of a 2005 exhibition at Kensington Palace.
Testino has said the princess was especially pleased with the photos because her sons loved them. "She said to me at the time [that] her children said it was the most 'her' they had seen," Testino has said.
No surprise, the princes have turned to Testino to chronicle milestones in their own lives; William's 21st birthday and again on Harry's 20th birthday, when the brothers were photographed clowning with their father, the Prince of Wales, during a family holiday.
Prince Charles is also fan of the photographer. He sat for Testino for Vogue pictorials in 2002 and again this year.
Testino, of course, can now rank himself the lead candidate to take the couple's official wedding portraits come 29 April 2011, when they say their 'I do's' at Westminster Abbey.
Want more of Kate Middleton? Check out the shocking value now placed on the sheer dress Middleton was wearing when she first met her Prince Charming.