To the world, she's one of the most captivating and enchanting royals.
Paul, who worked for the Princess of Wales from 1987 until her death ten years later, says there's no denying Kate Middleton simply adores her husband but the 35-year-old is lacking the same "magic quality" as her late mother-in-law.
"She's a lovely, lovely girl... she totally adores her husband, they're devoted to each other," the 59-year-old explained to 9 News.
“But that extra something, which you would call the ‘X-factor’, it's not there," Paul continued, before sticking the boot in one final time.
“I met Mother Teresa, she had it. Pope John Paul II had it. The Queen has it. Diana certainly had it. Kate doesn't.”
On Wednesday morning, Princess Diana's long-time confidante appeared on The Today Show and host Karl Stefanovic wasted no time questioning Paul on his comments.
“We have a lot to talk about this morning, but first I want to ask you about your comments about Kate Middleton... That’s way too harsh, isn’t it? How dare you even say that," Karl fumed.
“Well, because you see, standing beside someone like Diana I saw something so special and so magical that I can’t — I have never seen that,” a flustered Paul defended.
“How often have you stood beside Kate?” Karl quizzed.
“No, I haven’t. I don’t know Kate. I just see her the way you see her, through a lens,” Paul explained.
Paul tried to patch up the awkward exchange by saying his comments were taken out of context and Kate is a "beautiful lady" who "should be Queen of England."
But Karl wasn't impressed with Paul's answer.
“With respect, you are not taking anything out of context, you are saying it again and again and again that she doesn’t measure up to these other people in front of the camera. It is ridiculous,” Karl said.
Adding: "Why don’t you leave Kate alone? Do you have some sort of oxygen deprivation at the moment? I mean, come on. Leave her alone. She is a mother of two doing the very best she can. She is awesome."
WATCH: Karl grills Paul in the video player below. Post continues after...
Since Diana's death in 1997, Paul has muddied his ties with the Royal Family after doing several explosive interviews about the late Princess.
In 2002, he was charged with theft over Princess Diana's property but the case was dropped.
During a stint on Celebrity Big Brother in 2004, he labelled his one-time boss "lonely" and "desperate."
He also released a memoir in 2003 called A Royal Duty, which lifted the lid on life behind the palace gates.
Earlier this year Paul came out as gay and wed his long-time partner Graham Hooper.