Charles, who also has cancer, and his beloved daughter-in-law stood side-by-side on the Buckingham Palace balcony as a striking symbol of unity and defiance against the disease.
Both were diagnosed within a few short weeks at the start of the year and have grown closer as they support each other through their illnesses.
Royal insiders said Kate, 42, spoke personally to her 75-year-old father-in-law to let him know she would be attending Trooping the Colour her first public appearance since Christmas and he was "delighted" by the news.
Early photographs yesterday showed the Princess of Wales arriving at Buckingham Palace with her children, Prince George, 10, Princess Charlotte, nine, and Prince Louis, six, ahead of the birthday parade.
Kate looked typically stunning and stylish in a crisp white Jenny Packham dress with black details and matching wide-brimmed hat by Philip Treacy.
Pinned to her chest was the Irish Guards Regimental Brooch as their Colonel.
Her much anticipated appearance came less than 24 hours after the princess shared a deeply moving message about her ongoing cancer treatment.
She said in a statement on Friday that she was making "good progress" but had "good days and bad days" and admitted "I am not out of the woods yet".
Shortly after her arrival at the palace, she and her children appeared in the glass state coach to deafening cheers and cries from excited crowds in The Mall.
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