Kate McCann revealed story behind infamous Madeleine McCann photo

When three-year-old Madeleine McCann first disappeared 17 years ago, a series of images of the British toddler were quickly circulated across in an appeal for witnesses of her fate to come forward.

One photo of the Leicester tot - who was reportedly snatched from her bedroom while on a family holiday in the Portuguese Algarve in 2007 - which is still used by investigating officers to this day, sees Madeleine bearing a pink sun hat and holding a bundle of tennis balls closely in her arms.

This heartbreaking image of the happy youngster - as well as numerous others - has since been discussed in great detail by her mother Kate, who, with her husband Gerry, had been enjoying dinner at a nearby restaurant that fateful night.

Only when returning to the two bedroom Praia da Luz apartment - which Madeleine had been sharing with her younger twin siblings, Sean and Amelie - at around 10pm, did Madeleine's mum discover that the youngster was missing.

An investigation - which would later be branded the 'most reported on missing person's case in modern history' - was quickly prompted, which would continue for almost two decades and see millions of pounds invested.

Recalling her experience, Kate McCann subsequently published a book, titled 'Madeleine: Our Daughter's Disappearance and The Continuing Search For Her', which would touch on the initial appeal launched by police.

She also lifted the lid on the infamous photo of her little girl playing on a tennis court, which would be used by officers in a bid to locate her.

Kate explained what led to Madeleine's famous tennis ball photo. (PA)

"During Gerry’s tennis lesson, Madeleine and Ella came to the adjoining court with their Mini Club for a mini-tennis session," Kate wrote in the memoire.

"Jane and I stayed to watch them.

"It chokes me remembering how my heart soared with pride in Madeleine that morning. She was so happy and obviously enjoying herself."

Kate continued: "Standing there listening intently to Cat’s instructions, she looked so gorgeous in her little T-shirt and shorts, pink hat, ankle socks and new holiday sandals that I ran back to our apartment for my camera to record the occasion.

"One of my photographs is known around the world now: a smiling Madeleine clutching armfuls of tennis balls."

Madeleine's tragic disappearance has been the subject of countless documentaries and a handful of global conspiracy theories.

Earlier this year, on the 17th anniversary of the toddler's disappearance, Kate and Gerry shared a heartbreaking post on social media, commemorating the life of their little girl, who would now be 20 years old.

Madeleine would now be 20-years-old. (PA)

"It's 17 years since Madeleine was taken from us," the couple wrote. "It's hard to even say that number without shaking our heads in disbelief.

"Whilst we are fortunate in many ways and able to live a relatively normal and enjoyable life now, the 'living in limbo' is still very unsettling. And the absence still aches."

They continued: "Your support continues to encourage us and bolsters our strength to keep going.

"We know the love and hope for Madeleine and the will to find her, even after so many years, remains, and we are truly thankful for that.

"Thank you again for remembering Madeleine and all missing children."