Kate, 42, says she is in the "early stages" of treatment and has said she is "getting stronger every day by focusing on the things that will help me heal; in my mind, body and spirits".

Here's everything we know so far.

What treatment is the princess having?

The Princess of Wales has described the chemotherapy as "preventative".

She says in her message: "In January, I underwent major abdominal surgery in London and at the time, it was thought that my condition was non-cancerous. The surgery was successful. 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."

Kate says she has a "fantastic medical team who have taken great care of me, for which I am so grateful".

"As you can imagine, this has taken time. It has taken me time to recover from major surgery in order to start my treatment," says says.

The princess did not reveal the type of cancer she is having treatment for, or what stage her cancer is.

Reassuring George, Charlotte and Louis

The princess says it has taken her and her husband Prince William "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 says she has told 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".

'William by my side'

Praising William, Kate said having him by her side "is a great source of comfort and reassurance too. As is the love, support and kindness that has been shown by so many of you. It means so much to us both."

What is preventative chemo and how long does it last?

Thomas Moore, Sky's science correspondent, explains the usual procedure following many surgeries.

"They do tests on the cells and that's when they would have discovered that it was cancerous," he says.

"When they are talking about preventative chemotherapy, that is in case there are any other cells that were left behind after the surgery, to make sure those are not the seeds of a cancer that grows back."

He says it is hopeful that she has been diagnosed at a "fairly early stage" and there hasn't been any spread.

He explains that some chemotherapies can be "very difficult to take" and the side effects can be "brutal" - but that isn't always the case.

Moore explains chemotherapy normally takes between three and six months, with rounds of treatment where drugs are pumped directly into the veins. These drugs target cancerous cells - but also other cells like hair follicles.

"That is why we need to give huge understanding to Kate and what the family is going through.

"This is an awful lot to process. Cancer is still the diagnosis that no one wants to hear," he adds.

"Even though the treatments have got much better and the outlook is much better than it's ever been before, it's still the dreaded diagnosis."

When did the princess get her diagnosis?

Laura Bundock, Sky's royal correspondent, says Kate will have known "for some time".

The princess began the course of preventative chemotherapy at the end of February, but we don't yet know what kind of cancer she has or what stage it is - this is her "private medical information".

Bundock says there had been a "huge amount of pressure" on the Princess of Wales during her absence from the public eye.

The "void" created has been "filled with wild speculation", she says.

Bundock says it's clear the reasons Kate has decided to go public, and this is "very much connected" to her children.

PM's statement

"My thoughts are with the Princess of Wales, Prince of Wales, the Royal Family and in particular her three children at this difficult time," Rishi Sunak has said.

"The Princess of Wales has the love and support of the whole country as she continues her recovery.

"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.

"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."

Read more:
Kate's full statement
Live updates

Timeline of events leading to princess sharing her diagnosis

When did the Princess of Wales have surgery?

On Wednesday 17 January, Kensington Palace said the Princess of Wales had undergone a successful planned abdominal surgery.

At the time, it was reported the procedure was routine and the princess's condition was non-cancerous.

The palace said she would be off royal duties until after Easter while she takes time to recover.

The full statement read: "Her Royal Highness The Princess of Wales was admitted to The London Clinic yesterday for planned abdominal surgery.

"The surgery was successful and it is expected that she will remain in hospital for 10 to 14 days, before returning home to continue her recovery.

"Based on the current medical advice, she is unlikely to return to public duties until after Easter.

"The Princess of Wales appreciates the interest this statement will generate. She hopes that the public will understand her desire to maintain as much normality for her children as possible; and her wish that her personal medical information remains private.

"Kensington Palace will, therefore, only provide updates on Her Royal Highness' progress when there is significant new information to share.

"The Princess of Wales wishes to apologise to all those concerned for the fact that she has to postpone her upcoming engagements. She looks forward to reinstating as many as possible, as soon as possible."

Kate leaves hospital

On 29 January, it was revealed the Princess of Wales had been discharged from hospital.

Kensington Palace said in a statement: "The Princess of Wales has returned home to Windsor to continue her recovery from surgery. She is making good progress.

"The Prince and Princess wish to say a huge thank you to the entire team at The London Clinic, especially the dedicated nursing staff, for the care they have provided.

"The Wales family continues to be grateful for the well wishes they have received from around the world."

The palace had earlier said Kate was keen to "maintain as much normality for her children as possible" while she recovers.

It also said that while she "appreciates" the level of interest the public will have in her wellbeing, she hopes they will respect her desire for her personal medical information to remain private.