Prince William’s 21st birthday was filled with major controversy. The incident reportedly made the Duke of Cambridge upset.

In the Channel 5 documentary “Royal Scandals,” royal biographer Katie Nicholl detailed an incident involving an imposter. She said that comedy terrorist Aaron Barshak managed to sneak into Windsor Castle, where the Duke of Cambridge was celebrating his special day at that time.

The self-styled comedy terrorist was seen parading inside Windsor Castle dressed as Osama Bin Laden. He was also wearing a ball gown while making his way around the “Out Of America”-themed celebration.

“You can see from the footage Barshak walking up the Castle Gate, up to the castle all dressed up as a comedy terrorist, with a big black beard and his robes,” Nicholl said. Royal expert Daisy McAndrew added that it was quite difficult to monitor who was at the party back then because everyone was dressed in ridiculous outfits.

“However, the comedian was dressed in a pink dress and a fake beard, so he clearly didn’t fit with them,” McAndrew said. “He should’ve set security alarm bells going much earlier than he did,” she added.

To make matters worse, Barshak made his way to the stage while Prince William was giving a speech and gave the dad of three a kiss on the cheek. The gesture surprised Prince William and he asked, “Who the hell are you?”

“Security breaches don’t get any bigger than that” Nicholl said. “It was a huge royal scandal, suddenly all of the attention was off the birthday and what was essentially a massive security threat. Every senior member of the royal family was in attendance. Had that been a real-life terrorist, we’d be talking about a very different story today.”

Meanwhile, in her book “William And Harry,” Nicholl revealed that Kate Middleton was also in attendance at the party. While there, another woman took the pride of place next to the future king.

Nicholl said that Prince William seemed to be preoccupied with Jecca Craig, the daughter of British conservationist Ian Craig, at that time. Prince William and Craig first met each other in Kenya in 1998.

Prince William Prince William visiting Wallasey. Paul Townley/Flickr