The diagnosis came after Catherine underwent a planned abdominal surgery in January at the London Clinic. Officials did not immediately share details on the nature of the surgery, though they said it was successful and would likely keep her from public duties until after Easter.
On Friday, Catherine said that at the time, doctors believed her condition was non-cancerous. Tests after the surgery found that was incorrect.
“My medical team therefore advised that I should undergo a course of preventative chemotherapy, and I’m now in early stages of that treatment,” she said. “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.”
She did not say what form of cancer she was diagnosed with.
Catherine married Prince William in 2011. They share three children: George, 10; Charlotte, 8 and Louis, 5.
The former Kate Middleton had not been seen in public since Christmas, prompting rampant speculation online about her health, BBC News reported.
The scrutiny intensified earlier this month after Kensington Palace released the first photo of the princess following her surgery. The image, of Catherine and her children, later had to be rescinded because it had been manipulated without any notice.
On Friday, Catherine touched on her absence, saying that it has taken time for her to recover from the “major surgery” she underwent in January.
“Most importantly, it has taken us time to explain everything to George, Charlotte and Louis in a way that’s appropriate for them, and to reassure them that I’m going to be OK,” she said. “As I’ve 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 thanked the public for its support and asked for privacy for her family as she undergoes chemotherapy.
“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,” she said.
“At this time, I’m 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.”
Photos: Kate Middleton through the years Kate Middleton as a 5-year-old. (Middleton Family/Clarence House/Getty Images )
Photos: Kate Middleton through the years Kate Middleton, back row, third from left, is pictured in a tennis team photo during her time as a pupil at St Andrew's School in Pangbourne, England, where she attended from 1986 to 1995. (St Andrew's School - WPA Pool/Getty Images)
Photos: Kate Middleton through the years Kate Middleton and her father, Michael Middleton, before her graduation from the University of St. Andrews in June 2005. (Indigo/Getty Images)
Photos: Kate Middleton through the years Kate Middleton does some shopping during the Gatcombe Park Festival of British Eventing in August 2005. (Anwar Hussein/Getty Images)
Photos: Kate Middleton through the years Prince Harry, left, Prince William, center, and Kate Middleton cheer on the English team during the RBS Six Nations Championship match between England and Italy in 2007. (Richard Heathcote/Getty Images)
Photos: Kate Middleton through the years Kate Middleton takes part in a training session with The Sisterhood cross channel rowing team on the Thames in August 2007. (Max Mumby/Indigo/Getty Images)
Photos: Kate Middleton through the years Kate Middleton and Prince William announced their engagement in November 2010. ( Chris Jackson/Getty Images)
Photos: Kate Middleton through the years The Duke and Duchess of Cambridge attend the Calgary Stampede in 2011. (Samir Hussein/WireImage)
Photos: Kate Middleton through the years Kate Middleton, right, sits with her sister, Pippa Middleton in the Royal Box during the 2012 Wimbledon men's final between Roger Federer and Andy Murray. (Clive Brunskill/Getty Images)
Photos: Kate Middleton through the years Kate Middleton plays hockey with the GB hockey teams at the Riverside Arena in 2012. (Chris Jackson - WPA Pool /Getty Images)
Photos: Kate Middleton through the years Kate Middleton and Queen Elizabeth II enjoy a children's sporting event in 2012. (Phil Noble - WPA Pool/Getty Images)
Photos: Kate Middleton through the years Prince William, Duke of Cambridge and Kate Middleton, Duchess of Cambridge, depart The Lindo Wing with their newborn son, Prince George, at St Mary's Hospital on July 23, 2013, in London. (Photo by Chris Jackson/Getty Images) (Chris Jackson/Getty Images)
Photos: Kate Middleton through the years Kate Middleton waves her wand and casts a spell on the set used to depict Diagon Alley in the Harry Potter Films during the Inauguration Of Warner Bros. Studios in Leavesden in 2013. (Paul Rogers - WPA Pool/Getty Images)
Photos: Kate Middleton through the years Kate Middleton tries her hand at cricket during the countdown to the ICC Cricket World Cup on April 14, 2014, in Christchurch, New Zealand. (Joseph Johnson/Getty Images for ICC Cricket World Cup)
Photos: Kate Middleton through the years The Duke and Duchess of Cambridge and Prince Harry spearheaded a campaign called Heads Together in 2016. (Nicky J Sims/Getty Images for Royal Foundation)
Photos: Kate Middleton through the years The Duke and Duchess of Cambridge pose with Prince Louis after his birth in 2018. (Samir Hussein/WireImage)
Photos: Kate Middleton through the years Kate Middleton and Meghan Markle attend Wimbledon in 2018. (Karwai Tang/WireImage)
Photos: Kate Middleton through the years. Princess Charlotte arrives for her first day of school, with her brother Prince George and her parents the Duke and Duchess of Cambridge, at Thomas's Battersea in London on Sept. 5, 2019, in London. (Aaron Chown - WPA Pool/Getty Images)
Photos: Kate Middleton through the years The Duke and Duchess of Cambridge visit Badshahi Mosque during their royal tour of Pakistan in October 2019. (Owen Humphreys - Pool/Getty Images)
Photos: Kate Middleton through the years The Duke and Duchess of Cambridge enjoy some sports during a 2020 trip to Ireland. ( Charles McQuillan/Getty Images)
Photos: Kate Middleton through the years Kate Middleton, the Duchess of Cambridge, center, is flanked by Camilla, Duchess of Cornwall, and Sophie, Countess of Wessex, on the balcony overlooking the Cenotaph during the National Service of Remembrance on Nov. 14, 2021, in London. (Richard Pohle - WPA Pool/Getty Images)
Photos: Kate Middleton through the years Kate Middleton, left, chats with U.S. Open champion Emma Raducanu as they return to the LTA's National Tennis Centre for The Homecoming on Sept. 24, 2021 in London. (Chris Jackson/Getty Images for LTA)
Photos: Kate Middleton through the years The Duke and Duchess of Cambridge take part in a land yachting session on West Sands beach in May 2021. (Max Mumby, pool/Getty Images)
Photos: Kate Middleton through the years Kate Middleton tries her hand at archery during a 2021 visit to Wolverhampton, England. (Jacob King - WPA Pool/Getty Images)
Kate Middleton Through the Years SANDRINGHAM, ENGLAND - SEPTEMBER 15: William, Prince of Wales and Catherine, Princess of Wales, look at floral tributes, following the death of Britain's Queen Elizabeth, at Sandringham Estate on September 15, 2022 in Sandringham, England. (Photo Toby Melville - WPA Pool/Getty Images) (WPA Pool/Getty Images)
Kate Middleton Through the Years LONDON, ENGLAND - SEPTEMBER 19: Kate Middleton, Princess of Wales is driven down The Mall after the funeral for HM Queen Elizabeth II's funeral on September 19, 2022 in London, England. Elizabeth Alexandra Mary Windsor was born in Bruton Street, Mayfair, London on 21 April 1926. She married Prince Philip in 1947 and ascended the throne of the United Kingdom and Commonwealth on 6 February 1952 after the death of her Father, King George VI. Queen Elizabeth II died at Balmoral Castle in Scotland on September 8, 2022, and is succeeded by her eldest son, King Charles III. (Photo by Tom Jenkins - WPA Pool/Getty Images) (WPA Pool/Getty Images)
Kate Middleton Through the Years CARRICKFERGUS, NORTHERN IRELAND - OCTOBER 06: Catherine, Princess of Wales pictured during her visit to Carrickfergus on October 6, 2022 in Carrickfergus, Northern Ireland. Prince William, Prince of Wales and Catherine, Princess of Wales are on a one day visit to the province. (Photo by Charles McQuillan/Getty Images) (Charles McQuillan/Getty Images)
Kate Middleton Through the Years LONDON, ENGLAND - OCTOBER 13: Catherine, Princess of Wales plays boccia as she attends the 10th Anniversary Celebration of Coach Core at Copper Box Arena on October 13, 2022 in London, England. (Photo by Heathcliff O'Malley - WPA Pool/Getty Images) (WPA Pool/Getty Images)
Photos: Kate Middleton through the years Catherine, Princess of Wales attends the Christmas Morning Service at Sandringham Church on December 25, 2023 in Sandringham, Norfolk. (Photo by Stephen Pond/Getty Images) (Stephen Pond/Getty Images)