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.