The final rehearsal for the King’s Birthday Parade, known as the Colonel’s Review, has taken place in London.

This year, the Number 9 Irish Guards will troop their colour on June 15 with the King in attendance.

They rehearsed at Horse Guards Parade in front of an audience of 8,000 after receiving a letter from their colonel, the Princess of Wales, apologising for her absence and wishing them luck.

In the letter, Kate, who is undergoing chemotherapy after a cancer diagnosis, told the Irish Guards how “proud” she is to be part of the regiment.

The letter said: “I wanted to write to you to let you know how proud I am of the entire regiment ahead of the Colonel’s Review and Trooping the Colour.

“I appreciate everyone trooping the colour this year has been practising for months and dedicating many hours to ensuring their uniforms and drill are immaculate.

“Being your colonel remains an honour and I am very sorry that I’m unable to take the salute at this year’s Colonel’s Review.

“Please pass on my apologies to the whole regiment, I do hope that I am able to represent you all once again very soon.”

Lieutenant Colonel James Shaw, Brigade Major of Household Division, responded in a statement: “We totally understand, and our thoughts and best wishes are with Her Royal Highness The Princess of Wales.

“We were even more determined to make the Colonel’s Review the best yet, for her. We are so proud to have the Princess of Wales as the Colonel of the Irish Guards.”

Charles, who has recently returned to public-facing duties while receiving treatment for an undisclosed form of cancer, confirmed last week that he will be in attendance and inspect the soldiers from a carriage rather than on horseback as he has done previously.

During the parade on Saturday morning, the Irish Guards mascot, an Irish wolfhound named Turlough Mor but affectionately known as Seamus, was led across the square by a scarlet-coated drummer.

The dog was presented to the regiment as the soldiers lined up.

Kate was represented at the Colonel’s Review by General James Bucknall, who acted as the chief inspecting officer of the day in the princess’s absence.

Field Officer in Brigade Waiting Lieutenant Colonel James Coleby shouted orders to the regiment while riding a white horse named Onyx.

A mock-up of the royal carriage, emulating the one which will hold the King in a week’s time, arrived at Horse Guards Parade in time to reach the saluting base at exactly 11am.

A royal salute was given and the national anthem was played in full. The Household Calvary and the Mounted Band processed around the square with the carriage.

Horses marched in pairs in front and behind the carriage, with two divisions in front and two behind before the group finally filed off the parade and down the mall to prepare for the mock firing of a 41-gun salute, which will take place next Saturday.

A total of 250 musicians, 20 pipers, 240 military working horses and almost 1,000 dual-role soldiers of the British Army’s Household Division took part in the event on Saturday.