The award-winning novelist Hilary Mantel has sparked a row with a description of the Duchess of Cambridge as a "shop-window mannequin" with no personality whose only purpose is to breed.

During a lecture at the British Museum, the double Man Booker prizewinner said Kate appeared "gloss-varnished", with a perfect, plastic smile; this was in contrast, the writer said, to Princess Diana, whom she described as awkward and emotionally incontinent.

Mantel's remarks were made two weeks ago, during a lecture organised by London Review of Books, a month after her latest novel, Bring Up the Bodies, won the Costa prize. The lecture, entitled Royal Bodies, was on royal women under the public gaze across history.

Mantel said: "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 appears precision-made, machine-made: so different from Diana, whose human awkwardness and emotional incontinence showed in her every gesture."

Mantel, whose latest novels are set in the Tudor court, said she saw Kate becoming a "jointed doll on which certain rags are hung". She added: "In those days [Kate] was a shop-window mannequin, with no personality of her own, entirely defined by what she wore.

Mantel's comments came at an event organised by the London Review of Books

"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."

She also condemned Kate's first official portrait, by Paul Emsley, which was unveiled in January. The eyes were dead, she said, and the sitter wore "the strained smile of a woman who really wants to tell the painter to bugger off".

During the lecture, Mantel went on to question whether the monarchy is a "suitable institution for a grown-up nation", in a society that sacrificed royal ladies and allowed them to be entertainment.

A spokesman for Mantel told the Telegraph the speech was not a criticism but "remarkably sympathetic", with the author speaking about royal women as victims of their predicament. "It is a piece about appearance," he said. "It's about being trapped. It is about the performance, how the institution of royalty has to project and how it comes across."