Kate Middleton expressed how sorry she was for missing this year's Trooping the Colour traditional rehearsal.

The Princess of Wales, who is undergoing cancer treatment, decided to skip this weekend's rehearsal for Sovereign's annual birthday celebration. However, she penned a letter to the Irish Guards expressing her regrets for not joining them during their rehearsal on June 8.

"I wanted to write and let you know how proud I am of the entire Regiment ahead of the Colonel's review and Trooping of the Colour. I appreciate everyone trooping this year has been practicing for months and dedicating many hours to ensuring their uniforms and drill are immaculate," read Middleton's letter shared on X, formerly Twitter.

"Being your colonel remains a great honor and I am very sorry that I'm unable to take the salute at this year's Colonel's Review," she continued. "Please pass my apologies to the whole Regiment, however I do hope that I am able to represent you all once again very soon. Please send my very best wishes and good luck to all involved."

The future Queen signed the letter as "Colonel Catherine." The Irish Guards were moved by Middleton's thoughtfulness and shared the letter on their account on X. They also expressed their well-wishes for the royal mother of three.

"The Irish Guards were deeply touched to receive a letter from our Colonel, Her Royal Highness, The Princess of Wales this morning," the Irish Guards wrote. "We continue to wish Her Royal Highness well in her recovery and send Her our very best wishes."

Kate Middleton
Britain's Catherine, Princess of Wales smiles as she arrives to attend the Royal Variety Performance at the Royal Albert Hall in London on November 30, 2023. DANIEL LEAL/AFP via Getty Images

Middleton was appointed as the new Colonel of the Irish Guards in December 2022, taking over the role from her husband, Prince William, per Hello! Queen Elizabeth appointed the Prince of Wales for the role in 2011. It was Prince William's first honorary appointment in the Army, and he was the Irish Guard's first Royal Colonel, according to the palace.

Last month, the palace confirmed that Middleton would not be carrying out her usual Inspecting Office role at the traditional Colonel Review. Lieutenant General James Bucknall, KCB, CBE, stepped in on her behalf.

Royal commentators and experts were not expecting Middleton to join the royal family for this year's Trooping the Colour due to her health condition. Also, she has skipped several engagements, including the D-Day ceremonies, which convinced royal expert and historian Rafe Heydel-Mankoo that she would also miss the annual parade to honor the official birthday of the Sovereign.

Kate Middleton
Britain's Catherine, Princess of Wales arrives to attend the "Together At Christmas" Carol Service" at Westminster Abbey in London on December 8, 2023. HENRY NICHOLLS/AFP via Getty Images

"There's been no mention of her attending the D-Day ceremonies, although the Prince of Wales will be attending the Canadian beach Juno, and also at the Omaha Beach, where the international heads of state, but no word of the Princess of Wales attending that. So I think it's rather unlikely we'll be seeing her at Trooping the Colour," Heydel-Mankoo said.

Trooping the Colour, an annual ceremonial event performed on the Horse Guards Parade in London, United Kingdom, by regiments of Household Division to celebrate the official birthday of the British Sovereign, is set on June 15.

Kate Middleton
 Catherine, Princess of Wales poses for a selfie with a concertgoer during a walkabout meeting members of the public on the Long Walk near Windsor Castle, where the Coronation Concert to celebrate the coronation of King Charles III and Queen Camilla is being held this evening on May 7, 2023 in Windsor, England. Andrew Matthews-WPA Pool/Getty Images
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Kate Middleton, Princess of Wales, Trooping the Colour