Kate, the Princess of Wales, was diagnosed with cancer and is undergoing chemotherapy, she said in a video message released Friday, two months after she underwent "major" abdominal surgery.
The type of cancer has not been disclosed. According to Kensington Palace, Kate started a course of preventative chemotherapy in late February.
The announcement sparked an outpouring of support and comes as Kate's father-in-law, King Charles III, also undergoes cancer treatment.
"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," Kate, 42, said in the video, which was filmed at Windsor on Wednesday. "As you can imagine, this has taken time. It has taken me time to recover from major surgery in order to start my treatment."
The cancer was discovered in post-operative tests after her planned abdominal surgery in mid-January, she said.
Kate described the procedure as a "major abdominal surgery," and said in her video, "at the time, it was thought that my condition was non-cancerous."
The exact type of surgery she underwent has not been released.
"The surgery was successful," Kate said. "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."
At the time of Kate's surgery, Kensington Palace said her condition was non-cancerous. The palace has provided little information on her condition since.
"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," Kate 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 spirits."
Kate said having her husband Prince William at her side has been "a great source of comfort and reassurance."
"We hope that you will understand that, as a family, we now need some time, space and privacy while I complete my treatment. My work has always brought me a deep sense of joy and I look forward to being back when I am able, but for now I must focus on making a full recovery," Kate said. "I am also thinking of all those whose lives have been affected by cancer. For everyone facing this disease, in whatever form, please do not lose faith or hope. You are not alone."
Kate will return to official duties when she's cleared by her medical team, according to the palace.
Charles said he is "so proud of Catherine for her courage in speaking as she did," a Buckingham Palace spokesman said Friday.
The king and the Princess of Wales were both in the hospital at the same time in January. In the last few weeks, the king has "remained in the closest contact with his beloved daughter-in-law," the spokesperson said.
Charles and his wife Queen Camilla "will continue to offer their love and support to the whole family through this difficult time," the spokesperson said.
Prince Harry and Meghan, the Duke and Duchess of Sussex, said in a statement, "We wish health and healing for Kate and the family, and hope they are able to do so privately and in peace."
The Sussexes' comments come amid a yearslong rift between Harry and William.
MORE: Kate Middleton diagnosed with cancer: King Charles and more reactU.K. Prime Minister Rishi Sunak said in a statement his thoughts are with Kate and her family, adding, "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," he said. "I know I speak for the whole country in wishing her a full and speedy recovery and look forward to seeing her back in action when she's ready."
Friday's announcement follows reports that several staffers at the London Clinic, where Kate was admitted in January, are under investigation for allegedly trying to access the Princess of Wales' medical records.
The announcement also comes weeks after she endured a photo editing controversy.
Kate marked Mother's Day in the United Kingdom, March 10, by sharing a photo on X of herself smiling and surrounded by her three kids. The image was the first official photo of Kate the public had seen since December.
The photo was picked up by several international news agencies, including The Associated Press, Reuters, Getty Images and Agence France-Presse.
Those agencies later retracted the image, saying it may have been edited by the royals prior to its release.
MORE: Kate Middleton's public apology for editing family photo is rare for a royalThe Associated Press said it appeared "the source had manipulated the image in a way that did not meet AP's photo standards."
The next day, Kate said her Mother's Day photo was edited and apologized for any "confusion" those alterations caused.
"Like many amateur photographers, I do occasionally experiment with editing," she said in a statement on X. "I wanted to express my apologies for any confusion the family photograph we shared yesterday caused."
Meanwhile, Kate's father-in-law, King Charles, is also undergoing treatment for an undisclosed type of cancer.
The king's cancer diagnosis -- which came after he was treated for benign prostate enlargement -- was announced publicly by the palace on Feb. 5.
The type of treatment has not been revealed.
The prime minister told the BBC in February that the king's cancer was "caught early."
