The deepening divide between Prince Harry and his elder sibling, Prince William, presents a significant challenge for the heir to the throne.

Prince William, Prince Harry

(Photo: Toby Melville - WPA Pool/Getty Images)

According to a source within the royal circle, should the ongoing discord persist, it could come at a significant expense to the monarchy. This is because William, 41, may find himself without the invaluable support of a close and pivotal advisor.

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"They were really, really close and this is what makes it so sad that they're now so far apart," said Charles Rae. "When William becomes king, I always believed Harry would be one of his top advisers. I think that's out of the window."

Prince William and Prince Harry
(Photo: Toby Melville - WPA Pool/Getty Images)

The royal commentator and writer elaborated that Princess Diana, the brother's late mother, had always expressed her desire for her sons to maintain a strong bond and remain as close as they could be. "They had each other," he said to Britain's The Sun. "And it was them against the rest of the world but of course now it's not happening."

Over the years, the siblings have navigated through various challenges in their relationship, particularly after Prince Harry, 39, made the permanent decision to withdraw from active royal duties.

Prince William, Prince Harry

(Photo: DOMINIC LIPINSKI/POOL/AFP via Getty Images)

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Nevertheless, reports suggest that the Duke of Sussex reached out to his brother and his sister-in-law, Princess Kate Middleton, shortly before they received the news of her cancer diagnosis, as revealed by ITV Royal Editor Chris Ship in March.