Royal Biographer Ingrid Seward has detailed the King's relationship with his two sons, following Prince Harry's surprise interview with Good Morning America about the monarch's health.
Seward has revealed details of King Charles's upbringing of Prince William and Prince Harry in her latest book, 'My Mother And I'.
The memoir also details the British monarch's relationship with his late mother Queen Elizabeth II, and how he had "always longed to be hugged by her".
Seward told GB News that the King didn't develop a close relationship with his mother until his "later life", going into his fifties.
Ingrid Seward says King Charles was an 'indulgent father'
PA / GB News
The release of the book follows the Duke of Sussex's bombshell interview with American television, as he became the first royal to discuss the King's cancer diagnosis publicly.
Speaking to ABC's Good Morning America, Prince Harry told the reporter that he "loves his family" and hopes the diagnosis of his father's cancer can "bring the family together".
Discussing the royal's early relationship with his father and brother, before the royal rift separated them, Seward detailed the "close" bond Prince Harry and King Charles shared, as Prince William focussed on "mummy" Princess Diana.
She revealed that the King would "take Harry on trips" and the young Prince would "dress up" for the occasion.
Prince Harry spoke about King Charles's cancer on Good Morning America
ABC / Good Morning America
LATEST DEVELOPMENTS:
Seward told hosts Eamonn Holmes and Isabel Webster: "I think Charles was very, very close to his children, especially Harry actually, because William was very much wanting to be with Mummy.
"But Harry loved doing things with Charles. He loved going into the gardens at Highgrove and learning about plants. And he loved the military, loved horses. And as soon as he was able to, Charles would take him on little trips he could do with him.
"On one occasion Harry even went dressed up in sort of military uniforms."
Seward revealed the relationships changed after his marriage with Princess Diana "broke down", and she used her "weapon" in taking the children as a result.
Ingrid Seward says King Charles was 'closer to Prince Harry' than to Prince William
GB News
The biographer explained to GB News: "Charles was an indulgent father - he would organise to take them to Polo, and Diana would take them shopping."
Comparing King Charles's parenting to his mother Queen Elizabeth II's, Seward said her role as a mother as opposed to Queen was "taken away" when Charles was four years old.
She added: "Princess Elizabeth only had Charles till he was age four, before she became Queen, and then virtually her children were taken away from her.
"Not literally, but mentally they were, because she was so busy being Queen. She was only 26 and she entered this world in 1952 of such a man's world and basically looking after the children went very much on the back burner.
"But that was very much the norm for those aristocratic families in those days. They saw their children in the morning and then they saw them again in the evening."