Prince William, Kate Middleton celebrate Princess Diana's legacy in surprising move

Prince William and Kate Middleton have paid a special tribute to late Princess Diana as they celebrated International Youth Day on Monday.

The Prince and Princess of Wales shared adorable photos with surprising message on their official Instagram story to mark the day.

The couple wrote: "Celebrating International Youth Day. The @dianaaward's young people committed to social action, promoting human excellence and inspiring future generation."

In March, future King William celebrated 25 years of the Diana Award with a moving speech at the prize ceremony as he attended the event at the Science Museum in London where he also presented awards to 20 recipients.

William said: "She taught me that everyone has the potential to give something back; that everyone in need deserves a supporting hand in life.

"That legacy is something that both Catherine and I have sought to focus on through our work, as have the 50,000 young people who have received a Diana Award over the past 25 years."

The heir to the throne went on: "I am so proud to see this belief of my mother’s manifested in the amazing young people receiving the Legacy Award tonight."