The news comes after many weeks of speculation about the health of the 42-year-old future queen, who has not been seen on official duties since Christmas.
Kate, Princess of Wales, has revealed she has been diagnosed with cancer and is undergoing preventative chemotherapy.
In a personal message, the 42-year-old said planned abdominal surgery in January was successful and it was initially thought her condition was non-cancerous.
But tests after the operation found cancer had been present, she said, adding: “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.”
The mother of George, Charlotte and Louis said doctors had advised her to have a course of preventative chemotherapy and she is currently in the early stages of that treatment.
“It has taken me time to recover from major surgery in order to start my treatment,” she said.
“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 okay.
“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 spirits.”
She described having her husband Prince William by her side as a “great source of comfort and reassurance”, adding: “As is the love, support and kindness that has been shown by so many of you – it means much to us both.”
The princess’s chemotherapy began in late February, although it is unclear when it will end.