Kate, the Princess of Wales, says she is undergoing preventative chemotherapy after tests taken following major abdominal surgery in January revealed that 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 an unspecified but non-cancerous condition.
However, in a video message, Kate said subsequent tests had revealed cancer had been found.
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.”
King Charles revealed in February that he too was to have treatment for cancer, meaning he has had to postpone his public royal duties.
Buckingham Palace said on Friday that the King is proud of Kate for her courage in speaking about her treatment.
Charles is “so proud of Catherine for her courage in speaking as she did” and has “remained in the closest contact with his beloved daughter-in-law throughout the past weeks,” after they spent time in hospital together, a Buckingham Palace representative said.
Charles and his wife Camilla “will continue to offer their love and support to the whole family through this difficult time,” the spokesperson added.