"The Queen has the power to say what their title is," Williams told CNN.
"But in the case of names, it is more of an informal conversation.
"Of course they have such respect for the Queen that if she says 'I really don't like that name,' they'd definitely take that into account."
The most popular names in the royal family are Albert for a boy and Victoria for a girl.
The Duke and Duchess of Sussex are due to welcome their first child some time in April.
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