A portrait of a smiling Queen Camilla taken by her daughter-in-law the Princess of Wales has won a prize.

The image of Camilla wearing a blue and white floral dress and cardigan, sitting on a bench with a trug filled with pelargoniums in the gardens of Raymill, her Wiltshire retreat, has won cover of the year in the PPA Awards.

It appeared on the front of a special royal edition of Country Life and keeping the identity of Kate as the royal photographer behind the cover shot “totally secret” was “challenging”, according to the magazine’s managing editor Paula Lester.

Camilla had commissioned the photograph to mark her 75th birthday last year and while she was the guest editor of the magazine.

The PPA award judges said: “This cover was crucial in setting the tone and becoming the centrepiece for what became the fastest-selling issue in Country Life’s history.”

It was the first time her formal portrait had been taken by Kate, who was then duchess of Cambridge, while Camilla was still duchess of Cornwall.

Ms Lester said: “When Country Life’s editor Mark Hedges and I sat down with the Duchess of Cornwall to discuss the issue that she’d be guest-editing to mark her 75th birthday, we had no idea who she wanted to take the cover shot.

“The duchess said: ‘Yes, I’m thinking of asking Catherine’.

“Mark and I glanced at each other quizzically trying to think of a famous photographer called Catherine as she clarified: ‘Catherine Wales’.

“‘Ah’, we said in unison, with Mark adding: ‘Yes, Your Royal Highness, I think we can work with that’.

“It was kept totally secret, which was challenging. One day I received a message while at the hairdressers that the Duchess of Cambridge was about to call to discuss the shoot.

“‘Hello, Paula — it’s Catherine, the Duchess of Cambridge here’, she said. ‘I’m calling to ask what sort of images you are looking for on the front cover of the Duchess of Cornwall’s special edition of Country Life’.

“I explained that we were after an informal portrait of the duchess, that showed her personality and kind, empathetic nature, as opposed to a posed formal shot.

“The duchess said she would do her very best but implored me to let her know if the images were not suitable.”

Mr Hedges said: “The Princess of Wales took a set of wonderful images and it was difficult to choose the best.

“The royal editor had the final word and chose the picture The King liked best when he joined our final page-proof meeting.

“The award is an amazing accolade for what was already our most successful issue ever. The picture was used worldwide and continues to appear, and will take its place in royal photographic history.”

The PPA Awards recognise achievement across the UK specialist media industry.