The duchess had slipped in with her husband before the gallery opened – mindful of the media scrum at the actual unveiling, which included television crews from Germany and Russia. "It's just amazing, I thought it was brilliant," she said.
There is no tiara, gilt, red plush or ermine in thepainting, just a sober background, a plain, dark blue-green blouse modestly fastened up to the neck and a glint of one expensive earring.
The artist Paul Emsley, Glasgow-born, South Africa-reared and winner of the 2007 BP portrait prize, said he had faced one difficulty with the portrait. Kate, he said, was just too beautiful to make a good subject.
Artist Paul Emsley discusses painting the Duchess of Cambridge's first official portraitLink to this video
"I think any artist would agree that with an older face, with lines or wrinkles, or strong distinguishing features, it's easier to create a likeness. But with a genuinely beautiful face, it's harder to convey character."
That character was, he felt, friendly and interested in the progress of the work, which was made from dozens of close-up photographs and a few sketches. "She's so nice to be with," he said, "and it's genuine."
The duchess sat for the painting at the artist's studio in Bradford on Avon, Wiltshire, and at Kensington Palace last May and June. Her one request was that she should look like a natural human being, not a woman on official duty.
The original plan for an unsmiling face evolved into a gentle enigmatic smile – but no teeth. "There isn't a single open-mouthed portrait in the collection," Sandy Nairne, the gallery director, said. Any visitor who wants to inspect the enviably white royal teeth need only slip next door to the last days of the exhibition of royal photograps by Mario Testino, in which the Duke and Duchess of Cambridge are shown displaying an unfeasible number of them.
The portrait is not grovellingly flattering: the scale is so large that viewers do not have to peer closely to see faint dark shadows under the eyes unmasked by Touche Éclat, and the first hint of fine lines.
Experts were divided. The art critic Waldemar Januszczak was "disappointed". The duchess had been "let down" by "another pretty ordinary painting of a royal of the sort that we've really been churning out for the last few hundred years in Britain". The eyes lacked "sparkle" and "there's something rather dour about the face," he told BBC News. Robin Simon, British Art Journal editor, said simply it was "a rotten portrait".
But Richard Stone, who has painted most members of the royal family over three decades, including the late Queen Mother on no fewer than six occasions, thought it had "a lovely informality about it, … and a warmth". It was "jolly brave … [to] paint it well over life size, because that's extremely difficult".
That the duchess has commented approvingly is encouraging for Emsley. The Queen, whose monarchical duties include suffering the worst excesses of artistic licence on a regular basis, was notably silent on Lucian Freud's robust depiction of her with – to paraphrase one critic – a chin sporting a five o'clock shadow and a neck like a rugby prop forward.
The Duke of Edinburgh, meanwhile, exclaimed "Gadzooks!" when he saw the work of portrait painter Stuart Pearson Wright, who depicted him bare-chested, with a fly on his shoulder and seedlings growing from his finger. "Why have you given me a great schonk?" Philip demanded.
The duchess is the royal patron of the gallery, and her portrait was a gift to the collection from the retired old master art dealer Sir Hugh Leggatt.
Stephen Deuchar, director of theArt Fund charity, said Leggatt wanted to commission the portrait in memory of his old friend, the art collector and philanthropist Sir Denis Mahon, whose collection the charity manages.
"There is a tradition of portraits of princesses, which he was very much aware of, and he wanted to commission this image of her before the full weight of responsibilities of state and motherhood descended on her – and as it turned out, just in time."