London, UK - Is Prince William jealous of his brother Prince Harry's success?

It's no secret that the two sons of King Charles III aren't on the best terms at the moment, but the rivalry between the feuding brothers seems to run deeper than expected.

William, the heir to the British throne, is said to be not only disappointed with his younger brother after Harry's break with the royal family but also feels a "certain jealousy" towards the royal dropout.

The reported reason? Harry's successful Invictus Games, which launched in 2014 and have enjoyed international popularity ever since.

"But I think there was a degree of jealousy about how well it had gone," royal expert Robert Jobson revealed in an interview with ABC News. "I do think that William was surprised how much this had been such a success and how much money was being thrown into it and how many governments were getting involved."

The Invictus Games, a kind of Olympic Games for physically and mentally disabled soldiers, were Harry's "number one passion project." According to the expert, the fact that his brother's heartfelt cause had led to such a huge global success was a sour note for William.

Jobson himself called Harry's commitment to the project "impressive." The prince, who has since left the UK and moved to California, served in the army for ten years.

In his scandalous memoir Spare, the Duke of Sussex hinted that his plans for the Invictus Games made his older brother uncomfortable.

William had reacted "extremely irritated" when he first heard about Harry's idea for the extraordinary sporting event and complained that it would cut funds from their foundation, the 39-year-old claimed in his tell-all book.

In a later interview, the father-of-two also admitted that there had always been a kind of "competition" between him and his brother.

A competition that apparently continues to this day, despite the estrangement of the two brothers.