Britain's King Charles III on Saturday expressed his "heartfelt thanks" to well-wishers, in his first statement since his 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.

The note was published on the 75-year-old monarch's website and the royal family's official page on social media platform 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 organisations 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," he concluded, signing the letter "Charles R."

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 continuing some administrative duties and held his weekly audience with the prime minister via phone this week.

Charles 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 subsequent tests had identified "a form of cancer".

- Harry meeting -

British Prime Minister Rishi Sunak has said the condition was "caught early".

On Thursday, Charles's wife Queen Camilla said that her husband was doing "extremely well" under the circumstances.

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

He has generally enjoyed good health, barring injuries from polo and skiing.

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

The diagnosis has left William, 41, shouldering a heavy royal burden as his wife Catherine continues to recover from a recent abdominal operation.

William also thanked the public for their "kind messages" this week.

Charles's estranged younger son Prince Harry flew back to Britain to see the King on Tuesday, adding to this week's drama.

The pair had a 45-minute meeting at Charles's Clarence House residence in London before Harry flew back to his home in the United States.