Prince Charles has branded Prince Edward an “idiot” after a heated row involving Prince William. Details about the royal siblings’ alleged feud were discussed in a new documentary.

In Channel 5’s “Royals At War,” royal author Katie Nicholl recalled one of the most brutal clashes between the brothers. He said that the heated row took place when Prince Edward invaded the privacy of the Duke of Cambridge while he was still studying at St. Andrews.

“When Prince William was at university, and a very strict issue from the palace to the media had gone out, there was going to be a protocol when Prince William arrived after that, the press was to leave the Prince alone,” Nicholl said.

But, according to Nicholl, there was a group of cameramen that stuck around outside the school. When Prince Charles’ press secretary discovered that they were waiting for Prince William, he asked them to leave the premises.

According to royal biographer Penny Junor, one of the camera crew told Prince Charles’ press secretary that they have permission to be there because they work for Prince Edward. However, the instructions to leave Prince William alone when he arrives in school were clear as day.

Nicholl then said that Prince Charles immediately phoned Prince Edward and told him that he was an idiot. The heir to the throne was visibly upset of what his younger brother had done.

“The message was quite clear: ‘You have behaved inappropriately. Leave St. Andrews immediately,’” Nicholl said.

But despite Prince Charles and Prince Edward’s heated row from years ago, reports swirled that the former is usually more at odds with Prince Andrew than his other siblings. The documentary revealed that the rivalry between the Duke of Cornwall and the Duke of York has even resulted in Queen Elizabeth II picking a side.

Her Majesty’s former footman, Paul Burrell, claimed that Prince Andrew is the Queen’s favorite child. In the eyes of the monarch, Prince Andrew has never done anything wrong. Junor seconded Burrell’s thoughts and said that the Queen seems to have a blind spot for her third child.

Prince Charles Britain's Prince Charles kisses Lady Sarah McCorquiodale, while her sister Lady Jane Fellowes looks on at RAF Northolt Airport near London, 31 AUG, following the arrival of Charles and the casket containing the body of his former wife, Diana. MARTIN HAYHOW/AFP/Getty Images