London, UK - Britain's King Charles III on Saturday expressed his "heartfelt thanks" to well-wishers, in his first statement since the royal's shock announcement that he has cancer.

Buckingham Palace said Monday that Charles had begun treatment, but did not specify what type of cancer he has.

"I would like to express my most heartfelt thanks for the many messages of support and good wishes I have received in recent days," the king wrote in a message to the public.

"As all those who have been affected by cancer will know, such kind thoughts are the greatest comfort and encouragement," he added.

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The note was published on the 75-year-old monarch's website and the royal family's official page on X.

Charles added that it is "equally heartening to hear how sharing my own diagnosis has helped promote public understanding and shine a light on the work of all those organizations which support cancer patients and their families across the UK and wider world."

"My lifelong admiration for their tireless care and dedication is all the greater as a result of my own personal experience."

Britain's National Health Service reported this week that the king's diagnosis had sparked a surge in online searches for advice about the disease.

Charles has taken an indefinite break from public duties while he receives treatment and is currently staying at the royal country residence of Sandringham, in the east of England.

He is not thought to have prostate cancer, since after his recent hospital procedure for a benign prostate enlargement the palace said that "a separate issue of concern was noted," adding later tests had identified "a form of cancer".

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The diagnosis came just 17 months into his reign following the death of his 96-year-old mother, Queen Elizabeth II, on September 8, 2022.

Prince William, heir to the throne, has taken on most of the king's duties alongside Charles's sister Princess Anne and wife Camilla.

Prince Harry, whose relationship with the royal family is in tatters, flew back to Britain to see his father on Tuesday, adding to this week's drama. The pair had a 45-minute meeting before Harry returned to the US.