Princess Diana's ex butler Paul Burrell has said the Duchess of Cambridge will never measure up to his old boss because she doesn't have the Princess' "magic quality".
Mr Burrell, who worked for the Royal family from the age of 18 as a footman, spoke fondly of Diana and described the Princess as his "best friend".
But when discussing Kate with Australian news network 9News, he said she "doesn't" have "it".
He said: "I met Mother Teresa, she had it, Pope John Paul II had it, the Queen has it, Diana certainly had it, Kate doesn't.
"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."
His comments come after Channel 4 broadcast Diana: In Her Own Words on Sunday, which achieved 3.5 million views, making it the most-watched show this year.
The documentary, Diana: In Her Own Words, is centred on tapes recorded by her speech coach, ex-Coronation Street actor Peter Settelen, during vocal coaching lessons between September 1992 and December 1993 at Kensington Palace.
But the former butler, who worked for Prince Charles and Diana for 10 years from 1987, didn't agree with the airing of the tapes, which have divided critics.
Mr Burrell said: "I think anything which happens in the premise of four walls between two people, is private."
On Sunday, royal experts criticised the decision to broadcast the tapes, which have previously been partially aired on ABC in the US.
Penny Junor said the programme was "incredibly mawkish", Hugo Vickers labelled it as "unhelpful" and Robert Jobson said it was "about ratings".
Channel 4 previously ignored a public outcry and criticism from those who accused it of paying "blood money" for the "deeply personal" recordings.
Previously, a Channel 4 spokesman said: "We carefully considered all the material used in the documentary and, though the recordings were made in private, the subjects covered are a matter of public record and provide a unique insight into the preparations Diana undertook to gain a public voice and tell her own personal story.
"This unique portrait of Diana gives her a voice ... at a time when the nation will be reflecting on her life and death."