The announcement and a video statement from her was released at 6pm.
Immediately a shocked nation began sending goodwill messages on social media.
See below to find out what local MPs and groups have said following the Princess’s diagnosis.
1. Sussex MPs send well-wishes to Princess of Wales following news of cancer diagnosis
There has been a huge outpouring of support for the Princess of Wales across Sussex as she announced she was having treatment for cancer. Photo: Getty Images
2. Sussex MPs send well-wishes to Princess of Wales following news of cancer diagnosis
Andrew Griffith, MP for Arundel and South Downs, tweeted: “Our thoughts are with the Princess of Wales, the Prince of Wales, their three young children and the whole British Royal Family at this time.” Photo: UK Parliament
3. Sussex MPs send well-wishes to Princess of Wales following news of cancer diagnosis
Henry Smith, MP for Crawley, responding to Prime Minister Rishi Sunak’s statement, said: “I fully endorse the Prime Minister’s statement and wish good health to Princess Katherine. Particular thoughts for Prince William, George, Charlotte and Louis at an unsettling time, with the King’s condition too.” Photo: UK Parliament
4. Sussex MPs and groups send well-wishes to Princess of Wales following news of cancer diagnosis
Mims Davies, MP for Mid Sussex, said: "Wishing Her Royal Highness The Princess of Wales a speedy recovery. Our thoughts and prayers are with her and her family. A message of hope and support of others in such a difficult time-it’s clearly also time for her to have the time and space she needs with her young family." Photo: UK Parliament