Kate Middleton, Britain's Princess of Wales, on Friday revealed that she has been diagnosed with cancer and is undergoing a course of preventative chemotherapy.

The update from the 42-year-old royal after a "tough couple of months" comes after weeks of speculation over her health and recovery.

In a personal video message released by Kensington Palace, a weak looking Kate said her planned "major" abdominal surgery in January was "successful" and it was initially thought her condition was non-cancerous.

"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," Princess Kate said in her video message.

"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... It has taken me time to recover from major surgery in order to start my treatment. But, most importantly, 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,” she 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," she added, on a positive note.

She described having William by her side as a "great source of comfort and reassurance", adding: "As is the love, support and kindness that has been shown by so many of you – it means much to us both."

"For everyone facing this disease, in whatever form, please do not lose faith or hope. You are not alone," she added.

Kensington Palace said the video message was filmed on Wednesday by BBC Studios, the production arm of the BBC.

British Prime Minister Rishi Sunak said the Princess of Wales had shown "tremendous bravery" with her statement, wishing her a "speedy recovery".

"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," said Sunak.

He was alluding to weeks of swirling rumours on social media and damaging headlines about the recovery of the Princess, who has been off royal duties since her abdominal surgery in London on January 16. She was expected to return to formal duties after Easter next month, which will now be deferred indefinitely.

The news comes as her father-in-law, King Charles III, is also undergoing treatment for an unspecified form of cancer.