The Duke of Sussex has had a strained relationship with his family in recent years, particularly his father, the King, and his brother, the Prince of Wales.
Prince Harry flew to the UK to meet with his father in February following Charles' cancer diagnosis, and he last saw Prince William at their uncle Lord Robert Fellowes' funeral in Norfolk in August, but the brothers reportedly did not sit next to each other.
And despite Harry's visits to the UK in May for the Invictus Games tenth anniversary and in September, for the WellChild Awards, the Duke has not seen his father in-person since February.
For any kind of real reconciliation to take place between Harry and Charles, William has to be involved, according to royal biographer, Robert Hardman.
Speaking on the latest episode of HELLO!'s A Right Royal Podcast, Robert says: "You had both of them [Harry and Charles] in London at the same time doing stuff, and people were saying, well, why can't they talk? Why can't they get together? And the reasons are complex, but they're interesting.
"The first one is simply stress levels. People around the King know that if he does sit down with Harry, Harry's got an enormous amount he wants to unpack, a lot he wants to talk about and get off his chest. And that kind of conversation is going to be a potentially heated one.
"And the view is, now is not the time with the King. The poor guy's trying to get through his cancer treatment. So that was one reason."
Robert continued: "Another reason was that any discussion between the king and Harry has also got to involve William. You can't have the King and his younger son having a sort of bilateral, some sort of agreement, without Prince William having a say in it, because ultimately, he's going to be taking charge one day. And that relationship is obviously a rather more complex one."
Listen to the full episode here...
Harry has been embroiled in legal action over his security in the UK since his move to the US with Meghan in 2020.
The Duke has been granted permission to appeal against the dismissal of his high court challenge over a change to his level of personal security when he visits the UK.
Harry took legal action against the Home Office over the February 2020 decision of the Executive Committee for the Protection of Royalty and Public Figures (Ravec) that he should receive a different degree of taxpayer-funded protection while in his home country.
But as Robert writes in the updated version of his book, Charles III: New King. New Court. The Inside Story, £17 / $33.99 from Amazon, Harry's legal case puts the monarch in an awkward situation.
Quoting a senior constitutional expert and advisor to the family, Robert writes: "'Here you have the infelicitous situation where the King's son is suing the King's ministers in the King's courts. That is pulling the King in three directions.'"
He goes on to write that if Harry shares any information from private conversations he had with his father in court, it would be "serious legal jeopardy" and the case could collapse.