The Princess of Wales is the honorary colonel of the Irish Guards, who are leading King Charles’ birthday parade this year, but she didn’t take the salute at their rehearsal

Kate Middleton has shared her apologies for missing a major rehearsal for Trooping the Colour, King Charles’ birthday parade, as she continues to stay out of the spotlight amid her cancer diagnosis.

The Irish Guards shared a letter from the Princess of Wales, 42, in a post on X (formerly Twitter) on Saturday, June 8, where she wrote of “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 immactulate,” Princess Kate added in the letter.

She continued, “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. 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 Princess of Wales concluded the letter by writing, “Quis Separabit” and signing off as “Colonel Catherine.”

“The Irish Guards were deeply touched to receive a letter from our Colonel, Her Royal Highness, The Princess of Wales this morning. We continue to wish Her Royal Highness well in her recovery and send Her our very best wishes. Quis Separabit @KensingtonRoyal @ArmyInLondon,” the Irish Guards wrote alongside images of the letter.

Kate is the honorary Colonel of the Irish Guards, who are leading the charge in this year’s celebration (only one flag can be presented at a time, so the five regiments of the Foot Guards — Grenadier, Coldstream, Scots, Irish and Welsh — rotate who holds the honor annually). Princess Kate would normally have taken the salute at the Colonel’s Review for the Irish Guards — which took place on June 8, one week ahead of the big parade in London — but the palace announced on May 30 that she would not carry out the moment.

Queen Camilla and Kate Middleton at Trooping the Colour 2022. SAMIR HUSSEIN/WIREIMAGE

Lieutenant General James Bucknall, a former Commander of the Allied Rapid Reaction Corps who has attended every Trooping the Colour since 2009 when he was appointed Colonel of the Coldstream Guards, instead took the salute at the June 8 event.

Last year, it was Prince William who led the Trooping the Colour rehearsal as the Welsh Guards regiment, of which he is the honorary Colonel, took their turn.

Princess Kate has maintained a low public profile since announcing on March 22 that she has cancer and is undergoing chemotherapy. However, she was recently spotted out with her family as well as running errands solo, sources told PEOPLE.

Prince William, 41, said last month that his wife is “doing well,” though palace sources say she is not expected to return to work until she has been given the all-clear to do so by her medical team.

It remains unclear whether Princess Kate will join members of the royal family at the King’s birthday celebration on June 15. She has attended the staple on the royal calendar every year since marrying Prince William in 2011, with the exception of 2020 and 2021 when the event was scaled down due to the COVID-19 pandemic.

The Colonel’s Review isn’t the first time Kate has missed a major event with the Irish Guards this year. In March, before her cancer diagnosis was revealed and as she was recovering from abdominal surgery that took place in January, she did not attend their traditional St. Patrick’s Day parade. Despite her absence, the Irish Guards kept the Princess of Wales top of mind during the festivities. A source told PEOPLE ahead of the event that they were planning to give Kate three cheers at the end of their parade.

Kate Middleton attends the Irish Guards’ St. Patrick’s Day parade in 2023. ANDREW MATTHEWS/PA IMAGES VIA GETTY

Meanwhile, King Charles is set to attend Trooping the Colour on June 15 with a slight change from last year. Although the monarch, 75, previously appeared on horseback in the procession, he will ride in a horse-drawn carriage alongside Queen Camilla.

The King recently resumed a fuller schedule of royal duties, including public outings, amid his cancer treatment, which was revealed in February.

The royal family at Trooping the Colour on June 17, 2023. NEIL MOCKFORD/GETTY

Although the royal joked during a May 9 outing to the Royal School of Military Engineering that he was glad to be “out of my cage” after spending several months working primarily behind the scenes, royal outings were reduced after British Prime Minister Rishi Sunak announced on May 22 that a general election would be held in July. Buckingham Palace said the royals would postpone engagements “which may appear to divert attention or distract from the election campaign.” However, it was determined that Trooping the Colour would go on as scheduled.