According to the insider, there are a number of key reasons Charles and the boys don't get along.
Charles is said to dislike the fact that the boys are so popular with the public, due to their ability to break with tradition and connect with the public on a more personal level.
The source said Charles "hates" that the boys are openly emotive about the mental health issues they endured following the loss of their mother — a move that allowed the public to see them as less regal and more relatable.
"My mother had just died, and I had to walk a long way behind her coffin, surrounded by thousands of people watching me while millions more did on television," Harry said in an interview with Newsweek last June. "I don't think any child should be asked to do that, under any circumstances. I don't think it would happen today."
The source also noted that the younger royal generation's more informal way of doing things may also be a source of irritation for Charles.
For example, William, Kate, Meghan and Harry all call staff by their first names, which Charles does not.
Tom Bower, the author of Rebel Prince: The Power Passion And Defiance Of Prince Charles, an explosive tell-all about Charles, said the younger royals are not fond of Camilla.
"In public, they are all smiles, but behind the scenes they do not like Camilla and see little of each other," Bower told New Idea in April.
"This is partly because of what happened to their mother Diana, but also because Charles
spent so little time with them when they were younger."
He also shed some light on the rumoured rift, saying Charles believed William was "lazy and not doing enough", while William resented taking orders.
"William resented being told what to do and was not keen, which deepened the rift with his father. But the Queen and Prince Philip also told him that he and Harry had to increase their royal duties, and so they have done — as we are seeing now."
But lending credence to Clarence House's stance on the matter, there is still evidence of warmth between Charles and his sons.
Most recently, the Prince of Wales played a key role in Harry's wedding to Meghan Markle, stepping up to walk his daughter-in-law down the aisle at the last minute following a very public falling out with her father.
At the wedding reception, Charles gave a touching speech in which he spoke about feeding Harry and changing his nappy as a baby, and called him "my darling old Harry".
Harry on his father's support
In Diana, 7 Days, a 2017 BBC documentary recounting the events after Diana's death, Harry said his father provided support after the tragedy.
"He was there for us," Harry told the BBC. "One of the hardest things for a parent to have to do is to tell your children that your other parent has died."
Following Diana's death, the royal family was subject to public criticism for its perceived treatment of the princess, who had struggled to fit into the monarchy and deal with her husband's infidelity.
Some questioned the wisdom of having the young princes walk behind their mother's coffin as it processed through London past vast crowds of mourners.
Harry, however, said he was "very glad" to have been part of the day.
