King Charles will hold a party to celebrate the milestone on November 14 at Clarence House, in London. He extended an invitation to the Duke of Sussex, who has turned it down, The Sunday Times reported.
It comes two months after the prince reportedly refused to attend a memorial for the first anniversary of Queen Elizabeth II’s death.
It is believed the king and his son have not seen or spoken to each other in person since Queen Elizabeth’s funeral, last September.
Prince Harry attended the 70th anniversary celebrations for the king, who was Prince of Wales at the time.
During a BBC documentary to mark the occasion, the prince said his father was doing an “amazing job”. He also said he was thankful for the support Charles had given him and his wife Meghan, Duchess of Sussex.
The couple stepped down as senior royals in 2020 and moved to the US.
The following year, they were interviewed by Oprah Winfrey and provided insights into life in the royal family.
Their revelations escalated during the first year of King Charles’s reign, with the couple featured in a Netflix documentary.
In the six-part series, they shed light on their troubled life within the royal family and accused Kensington Palace of lying to protect Prince William, now Prince of Wales.
King Charles III's first year - in pictures
That was followed a month later by the release of Harry’s autobiography, Spare, and accompanying media appearances, during which he shared further painful revelations.
A friend of the king told The Sunday Times that when Charles "gets to the end of another successful work day and thinks of his family, there will always be a twinge in his heart about a problem that he hasn’t been able to solve yet".
“The current act is a challenging one, there are issues that aren’t resolved and there won’t be a rapprochement any time soon. There is the sense of a cooling-off period from the family that is under way after the aftershocks of the book and the interviews," the friend said.
“But that doesn’t change the king’s love for his son. He’ll never not invite his son to a family gathering, because that’s not who he is.”