Lady Colin Campbell has said the Duchess of Sussex's "graceless" antics would have "appalled" Princess Diana.
In a new interview, Lady Colin said Meghan's behaviour shows a "lack of understanding and appreciation of what being a royal is".
She told Woman's magazine: "Diana would be appalled, she had enough nous to know when to put on a show.
"She was very aware of how important it was to be gracious, while Meghan - and Harry to an extent - trot out the charm when it suits them, but otherwise are graceless.
"Her behaviour shows a lack of understanding and appreciation of what being royal is. I'm not sure who she thinks she is behaving this way."
Earlier this week, Lady Colin warned Meghan that she can't "hustle her way to the top" now that she's a member of the royal family.
The I'm A Celebrity star said she's "rooting" for Meghan but doesn't have a good feeling about her future if she doesn't change her ways.
Speaking on Good Morning Britain, Lady Colin said: "I'm rooting for Meghan but I have to say, the ride has been rather rockier than one would have hoped.
"I have an awful feeling that Meghan needs to understand things she doesn't yet understand.
Diana would be appalled, she had enough nous to know when to put on a show.
"Like there is a massive difference between being a celebrity, hustling your way from the bottom to the top, and she was a very proud hustler.
"She used to use the word hustle for herself and her first husband, very proudly.
"Being a royal; you don't hustle, the values, embodiment is completely different, she's sorted, she's got it, I think she needs to now embrace it and understand it's a new game in town."
Meghan has faced much criticism since becoming a royal.
Last month, Meghan and Harry faced backlash for making their son Archie's christening private.
They were also criticised by many for spending £2.4 million of taxpayer's money to renovate their home Frogmore Cottage.
Meanwhile, last month, reports claimed Meghan had banned people from taking photos of her at Wimbledon - which sparked a debate over her privacy demands.
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