Charles, 75, reportedly had lunch with his daughter-in-law Kate, before Kensington Palace released a moving video message from the Princess, in which revealed that she was undergoing "preventative chemotherapy".
Following the news, a Buckingham Palace spokesperson said that the King, who is battling his own undisclosed form of cancer, is "so proud of Catherine for her courage in speaking as she did".
WATCH: Princess of Wales reveals 'shock' cancer diagnosis
Kate, who first met Charles, when she began dating Prince William during their university years, has long shared a close bond with her father-in-law.
When asked about being introduced to the royal family in her engagement interview, the future Princess revealed: "I was quite nervous about meeting William's father, but he was very, very welcoming, very friendly, it couldn't have gone easier really for me."
MORE ON PRINCESS KATE HEALTH
With the King once describing Kate as his "darling daughter-in-law," their admiration and respect for one another is evident.
Take a look at Kate and Charles's sweetest moments through the years.
The Princess joined Charles and Camilla for a joint engagement at Trinity Buoy Wharf in London - a training site for arts and culture.
HELLO! understands that Kate was invited to join the visit by her father-in-law, Charles, in recognition of their shared interest and longstanding support of the arts and creative industries.
The Waleses carried out a rare engagement with Charles and Camilla in February 2020 as they visited the Defence Medical Rehabilitation Centre (DMRC) in Loughborough.
The King always seems to make Kate giggle in public.
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