A rare message from Kate Middleton has been made public as she continues her cancer treatment. 

In a letter to the British Army's Irish Guards regiment, the Princess of Wales expressed regret for missing the Colonel's Review, a rehearsal of the annual Trooping the Colour parade in honor of King Charles III's birthday. The preliminary event would have seen Kate, 42, take the salute as the unit's honorary Colonel for the first time.

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

The message continued, "Being your Colonel remains a great honour, and I am very sorry that I'm unable to take the salute at this year's Colonel's Review. 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 letter ended with the words, "Quis Separabit," which means "who shall separate (us)," and was signed, "Colonel Catherine."

Members of the regiment broke out in applause after the letter was read to them, as seen in a video shared on its X page.

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

Kate's absence from the rehearsal had been announced last month.  "This year General James Bucknall K.C.B., C.V.O will carry out the role of Inspecting Officer," a Kensington Palace spokesperson told NBC News May 30. "On behalf of HRH The Princess of Wales at Colonel's Review as she continues her recovery."

Kensington Palace has not commented on whether Kate, who has kept away from the spotlight since revealing her cancer diagnosis in a video released in March and last attended a public event last December, will make an appearance at the main Trooping the Colour event June 15.

In past years, the princess, husband Prince William and their three kids Prince George, 10, Princess Charlotte, 9, and Prince Louis, 6, have traditionally joined other royal family members to make a public appearance on a Buckingham Palace balcony during the annual parade.

The Prince of Wales recently shared a health update about his wife. Speaking to veterans at the 80th anniversary of the D-Day Landings in Portsmouth June 5, William said Kate was getting better and added, "She'd have loved to be here today."

Trooping the Colour celebrates the U.K. monarch's birthday every spring. The King, who turned 75 last November, is also battling cancer and has confirmed his attendance at the parade. However, he is expected to travel by carriage instead of horseback, multiple outlets have reported.

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Nearly two months after the Princess of Wales went public with her cancer diagnosis, a spokesperson for Kensington Palace said the royal has no immidiate plans to return to her public-facing duties.

"The princess is not expected to return to work," the rep said in a May 21 statement to BBC, "until it's cleared by her medical team."

On May 13, King Charles III bestowed the title of Colonel-in-Chief of the Army Air Corps to his oldest son, sparking controversy as many royal watchers believed the title would be more suited for Prince Harry, who trained and served in the military branch.

On April 26, nearly three months after sharing his cancer diagnosis, Buckingham Palace announced that Charles will return to public-facing duties. 

The queen consort attended the Royal Maundy Service on March 28 in place of King Charles III, making her the first spouse of the Monarch to continue the ancient tradition.

In a March 22 video message, the Princess of Wales shared that she'd been diagnosed with cancer after undergoing abdominal surgery in January.

"It was thought that my condition was non-cancerous. The surgery was successful," she said before noting that tests after the operation found cancer had been present. "My medical team therefore advised that I should undergo a course of preventative chemotherapy and I am now in the early stages of that treatment."

After photo agencies pulled the picture Kensington Palace shared of Kate since having her abdominal surgery on March 10, the Princess of Wales addressed claims the photo was doctored. 

"Like many amateur photographers, I do occasionally experiment with editing," she tweeted on March 11. "I wanted to express my apologies for any confusion the family photograph we shared yesterday caused. I hope everyone celebrating had a very happy Mother's Day. C."

Princess Diana's niece celebrated Mother's Day in the U.K. by sharing she and her husband Michael Lewis privately welcomed their first baby.

“It’s the joy of my life to be your mummy, little one. I love you unconditionally," she captioned her March 10 Instagram post. "Happy Mother’s Day to those who celebrate today."

After keeping up her full slate of engagements in the wake of her husband's cancer diagnosis, the palace cleared Camilla's schedule.

The Times pointed out March 2 that the 76-year-old didn't have any engagements on her calendar until March 11, when she'd be due at Westminster Abbey to observe Commonwealth Day.

The husband of Lady Gabriella Windsor and ex-boyfriend of Pippa Middleton, was found dead Feb. 25. Days later, a coroner's inquest found that he died by suicide.

While King Charles III was in the hospital for his benign prostate enlargement procedure, the royal family member was diagnosed with cancer.

"His Majesty has today commenced a schedule of regular treatments, during which time he has been advised by doctors to postpone public-facing duties," Buckingham Palace said Feb. 5. "Throughout this period, His Majesty will continue to undertake State business and official paperwork as usual.The King is grateful to his medical team for their swift intervention, which was made possible thanks to his recent hospital procedure. He remains wholly positive about his treatment and looks forward to returning to full public duty as soon as possible."

The Duchess of York's rep said in a statement on Jan. 21 that Sarah was recently diagnosed with malignant melanoma, an aggressive form of skin cancer. Several months prior, she underwent a single mastectomy to treat breast cancer.

On Jan. 14, Queen Margrethe II of Denmark made history as she officially abdicated the throne, handing the crown over to her son, now known as King Frederik the 10th. 

Kensington Palace announced on Jan. 17 that Kate Middleton underwent planned abdominal surgery and was set to remain in the hospital for 10 to 14 days.

"Based on the current medical advice," the Palace said, "she is unlikely to return to public duties until after Easter."

Amid Kate's recovery, Prince William postponed a number of engagements as he supported his family, including the couple's three children, Prince George, Princess Charlotte and Prince Louis.

Shortly after Kate's hospitalization was made public, Buckingham Palace shared that Charles "has sought treatment for an enlarged prostate."

"His Majesty's condition is benign and he will attend hospital next week for a corrective procedure," the statement added. "The King’s public engagements will be postponed for a short period of recuperation."

Princess Claire and Prince Felix of Luxembourg welcomed son Balthazar Felix Karl on Jan. 7, the first royal baby of the New Year!