Prince William and Kate Middleton don't often hold hands or publicly show affection, due to an unwritten rule that says royals must act professionally and keep their hands to themselves. This is why even the slightest sign of the couple being romantic in public or flirting with each other makes it to the headlines.

According to a lip leader, one such moment dates back to before the couple had tied the knot a decade ago. The occasion was Prince William's graduation from the Sandhurst Military Academy in December 2006, and his long-time girlfriend was there to celebrate the special day. She was accompanied by her parents, Carole and Michael Middleton, while British royals including Queen Elizabeth II had also attended the ceremony.

Kate, who was still four years and a break-up away from her royal life, could not help but gush at her boyfriend in a military uniform on the day. The then 24-year-old, who looked the perfect girlfriend of a future King in a red coat with black boots and a matching hat, apparently whispered "sexy" after spotting William on the grounds, reports the Mirror.

A lip reader claimed to Royalty's documentary "Royal Wives" that the future Duchess said, "I love him in uniform, he's so, so sexy."

According to royal expert Rebecca English, Kate's attendance at the event was extremely significant in her life as this was the moment she realised they would end up getting married despite his royal status and her being a commoner. Rebecca English told Channel 5's documentary "William & Kate: Too Good To Be True:" "We hadn't expected Kate and her family to turn up, and myself and a number of photographers and journalists were all standing on a press stand together. Suddenly out of the corner of her eye we saw Kate and her mother and father being ushered down to the front of the seating."

"I remember turning to the person next to me and saying that's it, it's a done deal. This woman is going to be our future Queen," English added.

However, to everyone's shock, Kate and William split just a few months after the event. The breakup only lasted for a few weeks though, and they got back together for good this time. Three years later, in October 2010, William finally got down on one knee and proposed to his girlfriend of seven years. They said their vows on 29 April, 2011 at Westminster Abbey in London, a decade since their first meeting at their college.

The Duke of Cambridge Prince William
The Duke and Duchess of Cambridge Prince William and Kate Middleton. Photo: Getty Images/Gareth Fuller Getty Images/Gareth Fuller