Amid reports that Prince Charles's relationship with his son Prince Harry has hit rock bottom, a royal expert has said that the situation might even get worse for the Prince of Wales and may be as damaging as his war with Princess Diana.

In his column for Mail Online, royal expert Richard Kay revealed a private joke that Charles and Harry shared while watching the fireworks after the latter's wedding with Meghan Markle. The heir apparently reportedly asked his son, "Who's paying for all this," to which Harry replied, "you are, pa."

According to Kay, the unusual exchange between the father-son duo was not uncharacteristic for Charles and Harry. Although Charles had an awkward and tense relationship with his elder son Prince William, he enjoyed a closeness with Harry that he treasured. Their relationship had improved in those years, as Prince Charles walked his son's bride down the aisle at their wedding, and used to publicly refer to Harry as "dear boy" and "darling boy."

However, their relationship has drastically changed since the May 2018 wedding, and they are barely on speaking terms now. Royal experts believe that Harry's recent clarification that he had expressed concerns about the Saudi billionaire who is at the centre of a cash for honours scandal involving Charles's closest aide, suggested that his relations with Charles haven't improved.

Kay writes, "To those close to Charles, it was seen as every bit as mischievous an intervention as Harry's criticism of his father as a parent...Criticising his father — and by implication the Queen — for the way he was raised is one thing. But the broadside he launched questioning the 'motives' of Dr. Mahfouz Marei Mubarak bin Mahfouz and highlighting his own 'concerns' about the Saudi businessman, is an implicit challenge to Charles's judgment."

Kay noted that "such rebuke, especially if it continues, could have an impact on the Prince as monarch as we edge closer to his reign," which is why those close to him fear the unravelling of his relationship with Harry could be potentially as "damaging" as that between him and Princess Diana.

A source close to the future King said, "There is a feeling among some of his long-term friends that Harry's outbursts, whether deliberate or unintentional, are chipping away at the Prince's authority. This matters, as the day he becomes King is now no longer somewhere over the horizon."

The insider added that Charles has been "hurt" and "baffled" by the recent actions of his younger child, but is desperate not to exacerbate matters and is avoiding any confrontation.

Charles, William and Harry
Prince Charles, Prince William and Prince Harry, during a commemoration ceremony to mark the 100th anniversary of the Battle of Vimy Ridge, in Vimy, near Arras, northern France, in April 2017 Reuters