Catherine, the Princess of Wales, has apologised for missing the final rehearsal before the Trooping the Colour parade in honour of King Charles III's birthday.

The princess, who is the honorary colonel of the Irish Guards, wished the regiment good luck as she recovers from cancer.

"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. 

The review is a dress rehearsal for the annual military parade held each June to mark the monarch's official birthday. 

The king will oversee the ceremony on June 15.

It is still unclear if Kate will attend the ceremony next weekend. 

She has undergone chemotherapy treatment 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, which was discovered after what she described as major abdominal surgery.

The king is also undergoing treatment for cancer but has eased back into public duties. 

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 260-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.

Some 8,000 people watched on Saturday as the Irish Guards in their bright red tunics and bearskin caps marched in formation at Horse Guards Parade in central London.

A military band played the national anthem and 240 horses marched as part of the exercise that also featured the regiment's mascot, an Irish wolfhound known as Seamus, draped in a red cape as he was led across the square by his handler.

The Irish Guards said on X that they were touched by the letter from Kate and wished her well in her recovery.

Kate succeeded her husband, Prince William, as colonel of the regiment in 2022 after he was named colonel of the Welsh Guards.

AP