The King has written privately to Joe Biden after the former US president was diagnosed with an aggressive form of prostate cancer.
It's understood that Charles, 76, 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.
It comes hours after the former US leader, 82, spoke personally about his diagnosis for the first time.
Mr Biden, who shared a photo with his wife, Dr Jill Biden, and the family's pet cat, wrote: "Cancer touches us all. 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."
He was diagnosed on Friday, with tests revealing the cancer had spread to his bones.
A statement issued via his office read: "Last week, President Joe Biden was seen for a new finding of a prostate nodule after experiencing increasing urinary symptoms.
"On Friday, he was diagnosed with prostate cancer, characterized by a Gleason score of 9 (Grade Group 5) with metastasis to the bone.
© Getty"While this represents a more aggressive form of the disease, the cancer appears to be hormone-sensitive which allows for effective management."
It added that Mr Biden and his family were "reviewing treatment options with his physicians," although no further updates were provided.
Support for the King
When the King was diagnosed with cancer at the start of 2024, Mr Biden expressed his concern.
He told reporters at the time: "I'm concerned about him. Just heard about his diagnosis. I’ll be talking to him, God willing."
© Chris JacksonHe later tweeted: "Navigating a cancer diagnosis, treatment, and survivorship takes hope and absolute courage."
Charles 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".
© GettyMr 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.
He served as President of the US from 2021 to 2025.
