By shedding her unremarkable home counties look of Jigsaw frocks and Monsoon jewellery for a wedding dress that was a perfectly executed homage to Grace Kelly, Kate Middleton instantly reinvented herself as a fashionista, front-row A-lister and perpetual Grazia cover girl to appear on rotation with Victoria Beckham, Kate Moss and Angelina Jolie.

This may prove to be a classic case of being careful what you wish for. By passing on the swaths of identical society dressmakers in favour of a high fashion heroine like McQueen's Sarah Burton, Kate was (perhaps unintentionally) making a bold and irreversible statement. She is now Fashion, daahling. And she'd better keep it up.

Kate's success and contribution to the British monarchy will now be measured not simply by what she does or says, but on what she wears. The ante was duly upped before her 2.75m train had made it to the altar. So much so that by the time the Prince of Wales's wedding party began and she reappeared in a simple white satin evening dress with diamante belt (also by Sarah Burton), style commentators on the BBC and Sky were already declaring it a bit route one after the faultless daytime creation. If Sophie Wessex were to appear in the same dress at a high-profile engagement she'd be lauded for raising her game. On Kate, it was now an anticlimax.

Like Princess Diana, who went from frumpy skirts and sweaters that wouldn't show stains from her kindergarten pupils' finger paints to an endless real-life catwalk show of high fashion looks from Yves Saint Laurent and Antony Price, Kate will now need to be immaculately dressed at all times if she's to maintain the love affair with the British press.

Kate's post-wedding day outfit of a simple blue Zara dress and cropped black blazer, though perfectly appropriate for a prep school parents' evening, will no longer cut it when the honeymoon period – literally and metaphorically – is over. By contrast, Fergie's wedding dress, all leg of mutton sleeves and whimsical motifs, was such a Sloaney montrosity that after a three-week period where little girls flocked to the local market for her signature oversized hair bows, our expectations hit the planet's core, never to rise again.

The pressure on Kate won't simply be about what she wears but about the provenance of her labels. By admirably choosing to showcase a British designer for the biggest media event of the year (and beforehand, for her Issa-clad engagement announcement), she has set herself up as an ambassador for British fashion.

Style critics are already questioning her decision to wear a Spanish brand on her first day as the wife of the future British monarch – laughably so, given that a frock from our own Topshop, manufactured in Asia and profited offshore, is arguably no more British than a sombrero. Kate will now be expected to fly the flag in Mulberry, Burberry, McQueen and Westwood, whether she fancies rocking a bit of Dior or not. Hardly a weighty cross to bear, perhaps, but when the British fashion industry and its workforce are struggling more than ever to compete in a global recession, one that a great number of ordinary people are relying on her to carry with flair.

Life could be a great deal worse, of course. No one is crying a river for a girl who has gone from an unexceptionally dressed commoner to a duchess with every fashion house at her disposal. But Kate's new status does add more scrutiny to a role that already carries a weight of huge expectation – one with which Diana struggled throughout her life. And as Kate's husband understands more painfully than anyone, that fairytale didn't end in a happily ever after.