The cancer was discovered in post-operative tests after her planned surgery.
LONDON -- Catherine, Princess of Wales, was diagnosed with cancer and is undergoing chemotherapy, she said in a video message released on Friday, two months after she underwent "major" abdominal surgery.
The type of cancer has not been disclosed. She started chemotherapy in late February.
The announcement sparked an outpouring of support and comes as her father-in-law, King Charles, King Charles, 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 abdominal surgery in mid-January, Kate said.
Kate described it as a "major abdominal surgery," and said in her video, "at the time, it was thought that my condition was non-cancerous."
The 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, William, the Prince of Wales, at her "side is 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.
King Charles is "so proud of Catherine for her courage in speaking as she did," a Buckingham Palace spokesman said.
The king and the Princess of Wales were both in the hospital at the same time in January. And in the last few weeks, the king has "remained in the closest contact with his beloved daughter-in-law," the spokesman said.
King Charles and his wife, Queen Camilla, "will continue to offer their love and support to the whole family through this difficult time," the spokesman 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.
TIMELINE: A look at Kate Middleton's cancer diagnosis, surgery, absence from public duties
U.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."
White House press secretary Karine Jean-Pierre acknowledged the announcement at the start of Friday's press briefing.
"Our thoughts are with the Duchess of Cambridge and her family members and friends during this incredibly difficult time and certainly we wish her a full recovery and I think it's important that we respect their privacy, especially at this time," Jean-Pierre said.
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.
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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.
The Associated Press said it appeared "the source had manipulated the image in a way that did not meet AP's photo standards."
Photo of Princess Kate retracted by news agencies over concerns of image manipulation
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."
MORE | Princess Kate's photo should have quelled the rumors. Editing it has done exactly the opposite
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.
King Charles III's cancer was 'caught early,' British PM says
The type of treatment has not been revealed.
The prime minister told the BBC in February that the king's cancer was "caught early."
2024 hasn't been a good year for Britain's royals so far.
First came the "royal health shock" headlines when palace officials announced that King Charles III and Kate, the Princess of Wales, both had health troubles. An absence of information about Kate's health and whereabouts then fueled an increasingly frenzied rumor mill that made its way from the fringes of social media to frontpages around the world.
Distrust of the monarchy only intensified after Kate admitted that she edited an official family photo, and palace officials have struggled to regain control of the narrative as conjecture and conspiracy theories persist.
Here is a timeline of recent events relating to the royal family:
Dec. 25, 2023: Kate attends Christmas service at Sandringham, the royal estate on the eastern English coast, alongside Prince William, their children and other members of the royal family. This is the princess's last public appearance.
Jan. 16, 2024: Kate, 42, is admitted to the London Clinic and undergoes abdominal surgery. The news isn't announced until the next day.
Jan. 17: Kensington Palace reveals that Kate is recovering from a planned operation. Officials say her condition isn't cancer-related but did not specify what surgery it was, only saying it was successful. They say she will remain in the private hospital for 10-14 days and be away from public view until after Easter.
Buckingham Palace announces on the same day that King Charles will be treated for a benign enlarged prostate.
Jan. 21: Prince Andrew's ex-wife, Sarah, Duchess of York, says she has malignant melanoma, a form of skin cancer.
Jan. 26: King Charles is admitted to a London hospital for a three-day stay for his prostate treatment.
Jan. 29: Kate and Charles are both discharged from the hospital. Charles is photographed leaving the hospital with Camilla and waving at well-wishers. Kate is not pictured leaving the hospital, but officials say she is "making good progress" and will continue her recovery at home.
Feb. 5: Buckingham Palace announces that Charles has cancer and will receive treatment. Officials did not say what form of cancer the king has.
Feb. 7: Prince Harry arrives in the U.K. from California to visit his father. He departs the next day.
Feb. 27: William pulls out of a memorial service for his godfather, the late King Constantine of Greece, at the last minute, due to a "personal matter." The palace declined to elaborate but said Kate continues to do well.
Later on the same day, Buckingham Palace says Thomas Kingston, the son-in-law of Prince Michael of Kent, has died at the age of 45. Prince Michael is a cousin of the late Queen Elizabeth II.
March 1: An inquest hears that Thomas Kingston died from a "traumatic head wound" on Feb. 25. A gun was found near his body at his parents' home.
March 4: A photo reportedly showing Kate riding in a car with her mother Carole Middleton in Windsor circulates on social media. It is published by some international news outlets but not in any of Britain's newspapers.
March 6: People magazine cites a royal spokesman as saying that William's "focus is on his work and not on social media."
March 10: Kensington Palace releases a photo of Kate surrounded by her three children to mark Mother's Day in Britain. The photo, the first official one since the princess underwent surgery, was retracted hours later by The Associated Press and other news agencies over concerns it had been digitally manipulated.
March 11: Amid a new round of speculation about her health sparked by the edited family photo, Kate issues an apology on social media for the "confusion" caused. She says she "occasionally experiments with editing" like many amateur photographers.
March 16: The Sun newspaper publishes a video reportedly showing a smiling and relaxed Kate shopping with William at a farm shop near their Windsor home.
March 20: The Information Commissioner's Office says it is investigating a report that at least one member of staff at the London Clinic tried to snoop on Kate's medical records while she was a patient at the hospital.
The Associated Press contributed to this post.