The Princess of Wales is attending trooping the colour in a carriage, along with her three children, Prince George, Princess Charlotte and Prince Louis, in her first public appearance since revealing her cancer diagnosis.
Catherine, who could be seen wearing white with navy details, was photographed earlier arriving at the event by car.
King Charles and Queen Camilla were riding in a carriage in their royal procession ahead of the Princess of Wales.
Ahead of the event, Kate had said in a statement: “I’m looking forward to attending the King’s Birthday Parade this weekend with my family and hope to join a few public engagements over the summer, but equally knowing I am not out of the woods yet.
“I am learning how to be patient, especially with uncertainty.
“Taking each day as it comes, listening to my body, and allowing myself to take this much needed time to heal.”
The Princes of Wales said she was making “good progress” but “there are good days and bad days”. “On those bad days you feel weak, tired and you have to give in to your body resting. But on the good days, when you feel stronger, you want to make the most of feeling well.” She added that her treatment will be ongoing “for a few more months”.
A Buckingham Palace spokesperson said: “His Majesty is delighted that the princess is able to attend tomorrow’s events, and is much looking forward to all elements of the day”.
The celebration has been held on the sovereign’s birthday for more than 260 years. At the event, Number 9 Company, Irish Guards, a regiment Kate represents as as colonel, will be trooping the king’s colour.
In a letter read to the regiment last week, the Princess of Wales, who is undergoing chemotherapy, wrote to the Irish guards to apologise for being unable to take the salute and wished them luck. Lt Gen Sir James Bucknall will take the salute on her behalf.
The king, who is also undergoing treatment for cancer, will not ride on horseback during the ceremony, but will instead review the parade from an Ascot landau carriage. He first attended his first trooping aged three, riding in a carriage with his grandmother, Queen Elizabeth the Queen Mother.