Kate, the Princess of Wales, wished the Irish Guards good luck on their final rehearsal on Saturday before the Trooping of the Colour parade in honour of the king's birthday.
Kate, who has been recovering from cancer, apologised for not being there in her capacity as the guards' honorary colonel.
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," she wrote in the letter shared by the Irish Guards on the social media platform X. "Please pass my apologies to the whole regiment, however I do hope that I am able to represent you all once again very soon.
The review is a dress rehearsal for the annual military parade held each June to mark the monarch's official birthday. King Charles III will oversee the ceremony June 15.
It's still unclear if Kate will attend the ceremony next week. She's been treated for an unspecified type of cancer she announced in March after speculation proliferated on social media about her wellbeing because she had not been seen in public for several months. She has revealed few details about her illness or treatment.
The king has eased back into public duties while continuing to undergo his own treatment for cancer. He attended commemorations this week for the 80th anniversary of D-Day, the invasion of Nazi-occupied Europe on June 6, 1944.
Trooping the Colour is a 460-year old tradition in which troops in full dress uniform parade past the king with their ceremonial flag, also known as their colour.
Charles is likely to travel to the event by carriage with Queen Camilla and is expected to watch the ceremony seated on a dais, rather than on horseback as he did last year.
The Irish Guards said they were touched by the letter from Kate and wished her well in her recovery.
