Testino used Diana music during photo shoots to capture royal engagement

The Peruvian, who created some of the most enduring images of William's late mother, took two pictures – one formal and the other more casual – of the couple at St James's Palace, looking very much in love and "brimming with happiness".

He has now revealed the relaxed portraits were taken with the help of music used while photographing Diana.

The tracks, from French singer Dalida, were played as the Prince and his fiancee posed together shortly after their engagement was officially announced in November.

Though members of the Royal Family are used to being photographed, even they must feel completely comfortable in order to look their best, he told US Vogue.

"The contrast of the contemporary informal music playing in the beautiful rooms with so much history caused a lot of laughter," he added.

Though William, Prince Harry and the Prince of Wales have all turned to Testino for pictures to commemorate milestones, the celebrated fashion photographer had never before captured Ms Middleton.

But he said he already felt "very confident" she would be an "easy sitter" because of her natural grace and he described taking her picture as "a real treat".

She had already developed a "perfect sense of royal responsibility", he told the magazine, adding: "Two people in love have a glow and a presence more powerful than anything else, and I wanted that to be the subject of the picture."

Two images were later selected for release by the couple, who chose Testino because he has worked with William on many occasions in the past.

The second in line to the throne's 21st birthday in 2003 was marked by the photographer who captured Harry's 20th birthday image in 2004, which showed the royal brothers joking with their father.

Charles was photographed by Testino in 2001 and again five years later to mark his first wedding anniversary to the Duchess of Cornwall.