Harry has launched multiple attacks on the Royal Family through TV interviews and his bombshell autobiography Spare.
"Would he privately admit he could have done some things better, or at least differently? Of course.
"But while I am sure they still don't agree on everything all the time, he and the Prince of Wales are firmly united now around a common shared interest, which is to keep the institution in a place where it can serve the nation.
"This was happening anyway as the late Queen got older, but there's no doubt what happened with Harry brought everything into sharp relief."
The source added that King Charles feels "great pain" over his relationship with Prince Harry and that there is "barely any level of communication" between the monarch and his son.