In a video message released Friday, Kate said the diagnosis came after she had underwent abdominal surgery in January. Her condition was initially thought to be non-cancerous, but later tests "found cancer had been present," she said.
Kate, 42, had been mostly absent from the public eye since her surgery in January, causing wild speculation about her health to swirl.
The shock diagnosis comes after King Charles III was also diagnosed with cancer following a procedure for an enlarged prostate.
Here's what we know.
What did Kate say?
Kate released a video Friday afternoon to provide an update on her health, two months after she stepped away from public life temporarily after her initial treatment.
"In January, I underwent major abdominal surgery in London and at the time, it was thought that my condition was non-cancerous," said Kate, who is married to the heir to the British throne, Prince William.
"The surgery was successful. However, tests after the operation found cancer had been present. 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."
She said her diagnosis had been a "huge shock" and that "William and I have been doing everything we can to process and manage this privately for the sake of our young family."
Kate and William have three children: Prince George, 10; Princess Charlotte, 8; and Prince Louis, 5.
A royal source told CNN that Kate began chemotherapy in late February, and that she and William had waited until now to reveal her condition because Friday was the day their children began their Easter school holidays.
"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 in the video message.
Kate did not say what type of cancer she had been diagnosed with.
Preventative chemotherapy is often used after surgery as an "adjuvant" therapy, to weaken the chances of cancer returning, Dr. Karen Knudsen, chief executive officer of the American Cancer Society, told CNN.
She had been expected to return to official duties after Easter, but will now postpone further work until she has been cleared to do so by her medical team, the source said.
CNN
