In a message posted on Instagram, he said the family would rally round Kate.

"Over the years, we have climbed many mountains together. As a family, we will climb this one with you too," he wrote on the social media site.

James Middleton is the younger brother of Kate and their sister, Pippa.

His message was posted shortly after the princess's announcement that she has been diagnosed with cancer and is undergoing preventative chemotherapy.

King Charles, who is also currently being treated for cancer, also shared a message of support for his daughter-in-law, with Buckingham Palace saying: "His Majesty is so proud of Catherine for her courage in speaking as she did.

"Following their time in hospital together, His Majesty has remained in the closest contact with his beloved daughter-in-law throughout the past weeks.

"Both Their Majesties will continue to offer their love and support to the whole family through this difficult time."

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