Kate, Britain's Princess of Wales, said on Friday she was undergoing preventative chemotherapy after tests taken following major abdominal surgery in January revealed cancer had been present.
Kate, 42, the wife of heir-to-the-throne Prince William, spent two weeks in hospital in January after having what her office said at the time was successful, planned surgery for a non-cancerous but unspecified condition.
However, in a video message, Kate said subsequent tests had revealed cancer had been found, but she said she was well and getting stronger.
"My medical team, therefore, advised that I should undergo a course of preventative chemotherapy, and I am now in the early stages of that treatment," Kate said in the video, which was filmed on Wednesday.
"This, of course, came as a huge shock, and William and I have been doing everything we can to process and manage this privately for the sake of our young family."
Royals with cancer
King Charles revealed in February that he, too, was to have treatment for cancer, meaning he has had to postpone his public royal duties.
Her office, Kensington Palace, said it would give no further details about the type of cancer. It said she was on a recovery pathway, and the preventative chemotherapy had begun in February.
After her operation, the palace said Kate would not return to official duties until after Easter, but her absence from public life has provoked intense speculation on social media.
She and William had wanted privacy about the cancer until their children, Prince George, 10, Princess Charlotte, 8, and Prince Louis, 5, began their school holidays, which started on Friday.
"It has taken me time to recover from major surgery in order to start my treatment. But, most importantly, it has taken us time to explain everything to George, Charlotte, and Louis in a way that is appropriate for them and to reassure them that I am going to be ok," she said.
"As I have said to them, I am well and getting stronger every day by focusing on the things that will help me heal in my mind, body, and spirit."
Kate has not appeared at a public event since she joined other members of the royal family for a church service on Christmas Day.
British Prime Minister Rishi Sunak said on Friday that Kate, the Princess of Wales, should be afforded the privacy to focus on her treatment and be with her family, following news that she was undergoing preventative chemotherapy for cancer.
"The Princess of Wales has the love and support of the whole country as she continues her recovery," Sunak said in a statement.
"She has shown tremendous bravery with her statement today. In recent weeks she has been subjected to intense scrutiny and has been unfairly treated by certain sections of the media around the world and on social media.
"When it comes to matters of health, like everyone else, she must be afforded the privacy to focus on her treatment and be with her loving family."