The royal mum-of-three, 42, shared the “huge shock” in a video message on Friday evening.
She vowed she is “getting stronger everyday” and chose to delay releasing the information to protect her children.
Prince George, 10, Princess Charlotte, eight, and Prince Louis, five, had already broken up from school and were with their parents when video was posted.
Sun editor Victoria Newton told BBC1’s Sunday With Laura Kuenssberg: “It was actually, I understand, that the Princess made the decision two weeks ago that she was going to do this public statement.
“The key thing for her as a mother, the priority was protecting those three children.
“She didn’t want them going to school, being asked even more than they already were.”
KING’S SUPPORT
A source claimed Kate had a heart-to-heart with King Charles, 75, at Windsor Castle before sharing the emotional announcement.
The monarch, who also has also been diagnosed with cancer, travelled from London for a private lunch with his “beloved daughter-in-law” on Thursday.
It takes huge bravery to do so, but her bravery will help millions of cancer patients and their families face this horrible disease.”
Dame Deborah James’s parents
Kate was keen to speak to the King about his battle and find out how he was coping.
It is believed the royals also discussed the mum-of-three’s video message and the princess was said to be encourage by the support her father-in-law received.
A source told The Times: “A factor in their decision to go public would have been the great warmth and affection received by the King as a result of his announcement.
“It meant a huge amount to him.”
They added how Charles helped Kate navigate sharing the message while “sensitively” handling how to protect her children.
Another source said previously of the intimate Windsor Castle meeting: “It is highly unusual for just the two of them to sit down together like this.
“The King had already been made aware that the woman he calls ‘my beloved daughter-in-law’ had cancer.
“They would have had lots to discuss and share because just weeks earlier the King had begun his course of treatment and dealt with announcing his diagnosis.
“The King left his lunch feeling very emotional.
“They are very close and he thinks of Catherine as his daughter.
Timeline of Kate’s health battle & recovery
JANUARY 16: Kate is admitted to the London Clinic for abdominal surgery
JAN 17: Kensington Palace announce the princess underwent surgery
JAN 18: William spends time at Kate’s bedside
JAN 23: The princess’ hospital stay passes one week
JAN 29: Kate leaves hospital
FEBRUARY 27: Prince William pulls out of service last-minute due to ‘personal matter’, sparking wave of unfounded conspiracy theories about Kate’s health
MARCH 4: Princess pictured in the car with mum Carole on the Windsor estate
Mar 10: Royal posts a sweet snap of her and the kids for Mother’s Day – then fans started spotting flaws, and massive agencies put out ‘kill notice’ on photo
Mar 11: Kate admits she edited photo, and is snapped in car with William
Mar 16: Royal fans spot the princess at farm shop near Windsor
Mar 17: Onlookers see Kate watching her youngsters playing sport
Mar 18: The Sun exclusively published video of Kate and Wills from two days prior
MARCH 22: Kate bravely reveals in an emotional video that she was given a shock cancer diagnosis.
“There is no doubt there is a lot they can share and can use each other for support during their own deeply personal cancer battles.”
It is understood Charles knew of the princess’ health update before he travelled from London for the lunch.
After Kate’s video message, Buckingham Palace said the King is “so proud of Catherine for her courage in speaking as she did” and remains in the “closest contact with his beloved daughter-in-law”
‘CAN’T BE TRUSTED’
Meanwhile a royal expert has claimed Kate and William did not share the news with Harry and Meghan Markle beforehand because they “can’t be trusted”.
An insider told the New York Post the Duke and Duchess of Sussex found out about the diagnosis along with the rest of the world.
The course claimed: “They had no idea, and only learned of the news around the same time the rest of the world found out.”
Following Kate’s announcement, the couple released a short statement.
More from Kate’s emotional message…
Harry and Meghan said: “We wish health and healing for Kate and the family and hope they are able to do so privately and in peace.”
Royal commentator Richard Fitzwilliams told The Sun: “There’s a deep rift between the brothers – that we know.
“We also know that they haven’t apparently spoken in many months, and the reason, of course, is that for some four years they’ve caused endless problems to the royal family and so they can’t be trusted.
“William hasn’t trusted Harry and Meghan and I mean with good reason.”
And, royal author Phil Dampier also told The Sun that Harry and Meghan were at the “bottom” of William and Kate’s priorities.
He said: “The Sussexes weren’t told about it because the relationship between William and Harry is at an all time low.
“When Harry came over to see his father after his cancer was diagnosed, William could have made time to meet him had he wanted to attempt a reconciliation, but he didn’t.
“It’s quite simple. I don’t think with all they’ve had to cope with in the last couple of weeks, dealing with Harry and Meghan would certainly have been bottom of their list.”
It comes after the pair’s Netflix “cheerleader” targeted Kate moments after her cancer diagnosis shocked the world.
Christopher Bouzy, who had a starring role in Harry and Meghan’s Netflix show, trolled the princess on X/Twitter 31 minutes after her video was released.
The tech mogul, 48, said Kate’s illness was stage-managed akin to “North Korean propaganda”.
Meanwhile, shamed Endgame author Omid Scobie, deleted a tasteless post on Friday showing an alarm clock set for 6pm — the time Kate made her cancer statement.
‘HUGE BRAVERY’
Following the shocking announcement, Princess Kate has received floods of support.
Well-wishers have been leaving flowers at Windsor Castle for the royal since revealing her news.
Dame Deborah James’s parents sent their love to the mum-of-three, praising her bravery and urging her to “hold on to rebellious hope”.
The family said they believe her statement would inspire many more people to get checked for cancer — and save lives.
Grovelling celebs who spouted vile Kate conspiracies are shameful bullies, says royal expert
VILE celebrities who dished out conspiracies about Princess Kate’s health should be “ashamed
Hugo Vickers, royal historian, told The Sun: “The conspiracy theorists, went in big time in the last few days.
“It says an awful lot about the mindset of these people. The high profile ones, but also the very irresponsible members of the general public who just sort of weighed in as well.
“It was extremely unattractive because, we all love the Princess of Wales, and she’s done a fantastic job.
“She’s really faultless in everything she’s done ever since she married Prince William.”
The expert added all the vile trolling and ‘Katespiracies’ would have “made things much more difficult” than they already were.
“I think quite simply, they should be feeling very ashamed of themselves,” he added.
“I think quite a few people have popped up and apologised, and rightly so.
“This is not something you should be joking about, or bonding about all sorts of absurd theories.”
“You will always have conspiracy theories because if you’re a conspiracy theorist, that’s what you do,” continued Mr Vickers.
“There are always going to be people coming up with theories and they always will so that you can’t stop.
“I think that anyone who pops up with some ludicrous theory about the princess would get very short shrift.
“And so I imagine that they will go to ground to be quite honest, because in a sense, they are bullies. They’re pushing it the whole time.
“And as somebody rightly said years ago, inside every bully, there’s a coward desperately trying to stay in.”
Heather, 66, and Alistair James, 68, told The Sun: “Kate, William and their family are in our thoughts, and we send all our love from our family to theirs.
“Deborah would be so proud. The Princess of Wales has shown incredible courage in speaking out so publicly about her treatment.
“It takes huge bravery to do so, but her bravery will help millions of cancer patients and their families face this horrible disease.
“What’s more, in being so open and honest, Kate will undoubtedly encourage many more people who are living with potential symptoms of cancer, to seek medical help. In doing so, she will save lives.”
Dame Debs died aged 40 in June 2022, after being diagnosed with Stage 4 bowel cancer.
She was just 35 when she was informed she had a cancerous 6cm mass in her bowl.
Like the princess, Deborah had young children, at the time they were nine and seven.
In her first interview with The Sun in 2017, Deborah said: “We had to face telling the kids, your first instinct is to protect them.
“We didn’t want them to think about it, but actually the best thing was just to be open with them.”
Dame Debs’ parents added: “We know if Deborah were here to send Kate a message, she would tell the Princess of Wales to hold on to rebellious hope.
“Deborah was a firm believer in the power of hope, and given how many times she beat the odds to survive, rebellious hope became her mantra.
“We have no doubt her positive mindset helped her keep going, and gave us five years of precious memories together.”
EASTER PLANS
King Charles has been resting after leaving Windsor Castle at 11am on Friday to go to his Highgrove House residence in Gloucestershire.
Queen Camilla has been his pillar of support and filled in at major engagements, including a visit to Northern Ireland.
While there she told well-wishers her husband is “doing very well” and “very disappointed he couldn’t come”.
Meanwhile Princess Kate has been recuperating at Adelaide Cottage, Windsor, with William, 41, and their children.
William hasn’t trusted Harry and Meghan and I mean with good reason.”
Richard Fitzwilliams
The couple were spotted at the Windsor Farm Shop last weekend.
It was the first time Kate had been seen since her abdominal surgery in January.
The pair will not attend the family Easter Sunday service which is expected to be held at St George’s Chapel at Windsor Castle.
It is understood the King is hoping to lead the family to the ceremony at Easter.
INGRID SEWARD: Prince William is a pillar of strength for Kate
STEADFAST when his family needed him, William has proved himself as a man and future monarch of strength and compassion.
While he might not have been present during Kate’s courageous video address on Friday — make no mistake, he has been by her side every step of the way.
William has stepped up with school run duties and provided a “great source of comfort and reassurance” to his wife.
But he has also been a comforting and reassuring presence for the public.
From leading members of the Royal Family at the Commonwealth Day Service to enjoying cheerful banter with youngsters on a youth centre visit, he hasn’t put a foot wrong.
Just this week he visited Sheffield to highlight his Homewards project to end homelessness in the UK.
Even then there were few chinks in his facade — but Kate was clearly on his mind.
“She needs to be sat here to hear this,” he smiled, when the discussion turned to the challenges of young families.
But ultimately he is only human.
To have your father and your wife diagnosed with cancer within such quick succession is heart-shattering.
It’s a remarkable insight into the character of this young man that he has been so incredibly brave.
Theirs is a very private turmoil which has been brought into the public arena by nature of their roles as future King and Queen.
William must feel the weight of enormous responsibility, both personally and for the monarchy, of which he is now the public face.
But he knows his most important role is that of dad and husband.
He will make that his priority.
This is a distinct difference from the ways of the past, where your royal role came before the role of spouse and parent.
Think back to when the Queen was young.
WILLS’ PRIORITY
It was a man’s world then — and she was forced to abandon her children in a way that William is determined not to.
William and Kate waited until the school term was over and their children were home before making their public announcement.
They needed to be prepared.
In this, they will have had the wholehearted support of Charles — who knows what it feels like to be brought up by nannies.
William was also acutely aware of the lack of his own father’s presence in his life, especially when things went wrong.
In 1991, he was accidentally hit over the head with a golf club when he was at school and Diana was having lunch in her favourite restaurant San Lorenzo.
The call came through that William had been bashed on the head and they thought he might die so it was very serious.
Both Charles and Diana rushed down to Ludgrove.
William was taken to the Royal Berkshire Hospital and then to Great Ormond Street Hospital and Diana went with him.
But ever a man of duty, Charles prioritised his engagements.
William is determined to do things differently.