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.
5. Sussex MPs and groups send well-wishes to Princess of Wales following news of cancer diagnosis
Maria Caulfield, MP for Lewes, said: "The whole country is sending love to the Princess of Wales. Certain parts of the media certainly need to look at their behaviour over recent weeks. Hounding a young mum to have to disclose her cancer diagnosis. Wishing her the speediest of recovery." Photo: UK Parliament
6. Sussex MPs and groups send well-wishes to Princess of Wales following news of cancer diagnosis
Caroline Ansell, MP for Eastbourne and Willingdon, said: "God Bless The Princess. So brave. I will write to Kensington Palace to send love from Eastbourne to her, The Prince and the children. Kate has asked for time and space to heal and she must have it, recent weeks have shown the ferocity of the attention she lives with, but when she is recovered, such messages of support will be seen, and I know, appreciated. She said exactly that in her extraordinary statement on Friday." Photo: London Portrait Photoqrapher-DAV
7. Sussex MPs and groups send well-wishes to Princess of Wales following news of cancer diagnosis
A spokesperson for Chichester Cathedral said: "We invite you to join us in prayer for HRH The Princess of Wales, her family as well as those affected by sickness." Photo: Staff