Paul Burrell insists Duchess Catherine will never measure up to Princess Diana because she doesn't have the same "magic quality".
The former butler of the late Princess of Wales - who died in a car crash in Paris in 1997 aged 36 - insists Prince William's wife simply doesn't have as much "charisma" as Diana, but admitted she is "devoted" to the Duke of Cambridge.
He said: "She's a lovely, lovely girl and if you get to meet Kate you will know she totally adores her husband and they're devoted to each other, but that extra something which you call the X factor, the magic quality, charisma ... it's not there."
The 59-year-old former footman also admitted Queen Elizabeth and Mother Teresa had something about them which stood out in contrast to Catherine, who was known as Kate Middleton before she married the prince.
Speaking on Australian news network 9News, he added: "I met Mother Teresa, she had it, Pope John Paul II had it, the queen has it, Diana certainly had it, Kate doesn't."
However, Paul was later slammed by 'Today' host Karl Stefanovic over his comments, and then appeared to backtrack, saying his words were taken "out of kilter".
Speaking on the Australian show, he said: "What I was trying to say was Diana was unique and irreplaceable and she had that something, which is charisma or magic. I'm not quite sure what it is.
"Kate is a beautiful young lady. She is gorgeous, she has never put a foot wrong. She has the love and support of her husband and they are the future of the royal family.
"I think we should support them and maybe my comment was out of kilter. I didn't mean it that way. What I meant was she is a safe pair of hands."