The King has written privately to Joe Biden to express his support after the former US president was diagnosed an aggressive form of prostate cancer.
Charles, who is still undergoing his own treatment for an undisclosed cancer, sent his best wishes to Mr Biden in the wake of his health news.
Mr Biden was diagnosed on Friday, with tests revealing the cancer had spread to his bones.
Mr Biden posted his first message on social media since the medical announcement, thanking people for their love and support on Monday.
“Cancer touches us all,” he wrote
“Like so many of you, Jill and I have learned that we are strongest in the broken places. Thank you for lifting us up with love and support.”
The King hosted the then-American leader for tea and talks at Windsor in July 2023, with a spokesman for the monarch saying the relationship between the pair had been “extremely cordial”.
Mr Biden also attended Queen Elizabeth II’s funeral in 2022, and, in 2021, he and Charles, then the Prince of Wales, held a bilateral meeting during the Cop-26 summit in Glasgow.
When the King was diagnosed with cancer at the start of 2024, Mr Biden expressed his concern.
Mr Biden, who is six years older than the 76-year-old King, told reporters at the time: “I’m concerned about him. Just heard about his diagnosis. I’ll be talking to him, God willing.”
He later tweeted: “Navigating a cancer diagnosis, treatment, and survivorship takes hope and absolute courage.”
Last week, the King spoke candidly about his own cancer treatment saying he would like to think he is on the “better side” of his journey.
Charles made the comment to a former cancer patient he met during an away-day with the Queen to Bradford.
Prostate cancer is assigned a Gleason score, which ranges from two to 10 and indicates how abnormal the cancer cells are when compared to healthy tissue under a microscope.
Mr Biden’s office reported his score was nine, suggesting a highly aggressive form of the disease.
“While this represents a more aggressive form of the disease, the cancer appears to be hormone-sensitive, which allows for effective management,” a statement from Mr Biden’s office read.
