Prince William did something that made Kate Middleton's 27th birthday not among her best memories, according to a report.

When the Princess of Wales was about to celebrate her 27th natal day, many expected the Prince of Wales to propose to her. However, she was forced to bid goodbye to him, and she was left "crushed" after Prince William decided to start his career in the Royal Air Force (RAF) in a bid to have extra time and freedom before becoming a full-time working royal, according to royal biographer and commentator Katie Nicholl in her 2013 book "Kate: The Future Queen."

"With that single decision, Kate's life was turned upside down. She had expected a proposal once William had completed his military training, but instead, she was forced to accept that there would be no engagement any time soon. Kate would have to wait even longer," Nicholl wrote, per the Mirror.

Prince William and Kate Middleton's Royal Wedding
 Their Royal Highnesses Prince William, Duke of Cambridge and Catherine, Duchess of Cambridge journey by carriage procession to Buckingham Palace following their marriage at Westminster Abbey on April 29, 2011 in London, England. Sean Gallup/Getty Images

"Was Kate prepared to join him? She supported his desires and ambitions, but privately, she was crushed. William was asking her to be an army wife -- without the nuptials. Her frustration was understandable."

In the weeks that followed, the Duke and Duchess of Cambridge spent some quality time in Scotland, where they celebrated the New Year. However, when her birthday came around, the heir apparent also started the next stage of his life. Nicholl claimed it was difficult for Middleton because she felt left behind as their pals were already settling down and starting their own families.

"For Kate, it was a step backwards. It seemed that everyone was getting married except for her. As she neared her 27th birthday, Kate's plan to be married with children by the time she was thirty was looking increasingly unlikely," Nicholl continued.

Prince William, Prince of Wales and Catherine, Princess of Wales, Kate Middleton
Prince William, Prince of Wales and Catherine, Princess of Wales visit the Indian Streatery, an authentic, family run independent Indian restaurant based in the city centre during their visit to Birmingham on April 20, 2023 in Birmingham, England. Arthur Edwards - WPA Pool/Getty Images

In related news, nearly two decades ago, Prince William and Middleton's first couple photo was released. Daniela Elser, a royal correspondent for News.com.au, revisited the snap, which was reportedly captured by either of Middleton's parents. The shot was taken when they had just graduated from the university and featured them wrapping their arms around each other, looking "relaxed, delighted, affectionate."

Elser also recounted Middleton's blunt response when she was told she was lucky to be going out with the future king.

"There is a story that goes around semi-regularly, that in the aughties when William and Kate were in their 20s and dating, that someone was enthusing about how lucky she was to have snaffled the affections of a prince only for Kate to reply, 'he's lucky to be dating me,'" Elser wrote.

Prince William and Kate Middleton's Royal Wedding
Their Royal Highnesses Prince William, Duke of Cambridge and Catherine, Duchess of Cambridge wave on the balcony at Buckingham Palace during the Royal Wedding of Prince William to Catherine Middleton on April 29, 2011 in London, England. John Stillwell-WPA Pool/Getty Images
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