Prince Charles
Prince Charles never thanked Robert Higdon for his service. Pictured: Prince Charles meets residents of The Guinness Partnership's 250th affordable home in Poundbury on May 8, 2015 in Dorchester, Dorset. Getty Images/Ben A. Pruchnie

Prince Charles was ungrateful to one of the men who worked hard for him.

According to Tom Bower, the Prince of Wales relied heavily on two men Robert Higdon, the head of his charity foundation in America and his valet Michael Fawcett. However, the gentlemen "disliked each other intensely."

The investigative author noted that Fawcett knew how to please the future king. During Prince Charles fund-raising dinners, he would be standing behind the royal waiting for any signal that he needed something. The Duke has a great relationship with Fawcett, but not so with Higdon.

"Charles and I had a dysfunctional relationship. He would phone on Thanksgiving and Sundays, which was disturbing. We did laugh a lot. But Charles never said thank you," Higdon said, according to Bower.

Meanwhile, Prince Charles trusts Fawcett. In fact, the latter's opinion matters to the Duke of Cornwall.

According to Bower's write-up, there was a time when John Studzinski, a generous American-born investment banker was invited to a lunch at St. James' Palace. Studzinski delivered a speech about raising funds for the homeless.

Afterward, Prince Charles reportedly told an organizer that "John shouldn't have made the address because Michael says that he doesn't give me enough money."

On the other hand, Higdon arranged for donations from Americans and supervised their visits to the royal. But according to him, Fawcett was difficult to work with.

"He not only provided bad food and horrible sweet German wine but deducted the cost of the food from my raised money," Higdon said about Fawcett. "And he organised the dinners like a Barnum & Bailey circus."

"He's one of the most horrible people I've met in my life," Higdon added.

According to Higdon, despite his conflict with Fawcett, "the Boss and the Blonde kept me because they knew the money I was bringing in." However, he was eventually fired in 2011. Prior to that, according to Higdon, Prince Charles' accountant Leslie Ferrar approached him.

"You know, Robert, people are very uncomfortable about you and your relationship with Prince Charles," Ferrar told Higdon.

Higdon's familiarity with Prince Charles reportedly crossed the line because he acted like a friend when he was just a servant. According to Bower, Higdon "remains bitter about Charles's lack of gratitude for all the millions he raised."

Higdon was also disappointed with Prince Charles' staff. In fact, he described them as "mean, vicious...the most horrible people I've worked with."

In related news, Bower said in his book "Rebel Prince: The Power, Passion and Defiance of Prince Charles" that Fawcett sold seats to access Prince Charles. The wealthy donors were enticed to give huge donations to the prince's charities to sit beside him in banquets.

The investigative author also wrote that the Prince of Wales was accused of dodging tax to pay his staff. When the royal was questioned by the Public Accounts Committee about the capital gains tax exemption in his duchy estate in 2013, Prince Charles reportedly used the Human Rights Act to prevent anyone from accessing his tax returns.