Prime Minister David Cameron has defended the Duchess of Cambridge, saying author Hilary Mantel was "completely wrong" to compare her to a "shop-window mannequin".
Mantel said at the London Review of Books Lecture the duchess was "gloss varnished" with a "plastic smile".
Mr Cameron added Mantel "writes great books" but "what she's said about Kate Middleton is completely misguided".
Her speech was titled Undressing Anne Boleyn, referencing royal women.
Mr Cameron added: "What I've seen of Princess Kate at public events, at the Olympics and elsewhere is this is someone who's bright, who's engaging, who's a fantastic ambassador for Britain.
"We should be proud of that, rather than make these rather misguided remarks."
Mantel said at her speech at the British Museum on 4 February that in contrast to Diana, Princess of Wales, the duchess appeared "machine-made".
"Kate seems to have been selected for her role of princess because she was irreproachable: as painfully thin as anyone could wish, without quirks, without oddities, without the risk of the emergence of character," she said.
"She appears precision-made, machine-made, so different from Diana whose human awkwardness and emotional incontinence showed in her every gesture. Diana was capable of transforming herself from galumphing schoolgirl to ice queen, from wraith to Amazon."
Mantel, whose Booker prize-winning novels, Wolf Hall and Bring Up The Bodies - which chart the rise and fall of Thomas Cromwell, a powerful minister in the court of King Henry VIII - said she saw Kate becoming a "jointed doll on which certain rags are hung".
"These days, she is a mother-to-be, and draped in another set of threadbare attributions.
"Once she gets over being sick, the press will find that she is radiant. They will find that this young woman's life until now was nothing, her only point and purpose being to give birth.
"Presumably Kate was designed to breed in some manners," she added.
'Back off and not be brutes'
Mantel's most recent prize was the Costa Book of the Year Award for Bring Up The Bodies.
She rounded off her speech by saying: "It may be that the whole phenomenon of monarchy is irrational, but that doesn't mean that when we look at it we should behave like spectators at Bedlam."
She added that although beheadings of "royal ladies" no longer take place, "we do sacrifice them, and we did memorably drive one to destruction a scant generation ago".
The lecture has sparked some backlash from the British press, with the Daily Mail calling it "an astonishing and venomous attack" on the duchess.
Labour leader Ed Miliband also voiced his views on the speech, telling the BBC: "These are pretty offensive remarks, I don't agree with them.
"Kate Middleton is doing a brilliant job in a difficult role. She's a huge asset to the country. She deserves our support in the role that she's playing."
However, Mantel received support a a surge of interest from people posting on the social network site, Twitter.
There was also support for the duchess, who visited an addiction treatment centre in London for which she is a patron, on Tuesday morning.
Nick Barton, who heads Action on Addiction, defended Catherine, describing her as an "intelligent" woman.
Asked for his reaction to Mantel's speech, he said: "I don't think it's for me to comment on that kind of stuff. I speak of what I know - somebody who wants to help, is helpful and genuinely interested and is intelligent," he said.
Last week, St James's Palace criticised Italian gossip magazine Chi after it published photographs of the pregnant duchess in a bikini, taken while she was on holiday with Prince William on the Caribbean island of Mustique.
But its editor, Alfonso Signorini, defended the use of the photos, saying they were "photos of a young couple in love" and were not an invasion of privacy.
Mantel has declined to comment on her speech.
St James' Palace has also declined to comment on behalf of the Duchess of Cambridge.