Katy Guest: So sorry Sienna. Big pants are back

When it comes to knickers, your granny knows best

Something suspicious is happening in celebrities' undergarments, and for once it has nothing to do with Russell Brand. All over the world, pants have been up. Some people wonder if it is due to the sudden cold snap, but I blame Britney Spears (why fight fashion?). Too many flashes of supermum's undraped naughty bits have famous ladies in a panic. Now it is not enough just to wear knickers: one has to be seen to be wearing them, too.

It started, as these things often do, with Sienna Miller. Remember her? The on-off girlfriend of Jude Law, who recently said that she wants people to respect her for her acting and not keep banging on about "what I'm wearing or my relationship". To this end she turned up for the New York premiere of her new movie, I forget which, with barely a stitch on. To prove her acting ability, she offered an avant-garde mime of one of those dreams where you get to school and suddenly realise that you've forgotten your skirt. In thick black tights and shiny nylon knickers, she brought tears to her audience's eyes.

Where fashion leads, Katie Holmes is sure to follow, and last week she did so triumphantly. Stepping from her car at Paris Fashion Week, this elegant young mum showed Britney how it should be done. In an assured movement, Katie kept both knees together, swept aside her billowing Armani dress and revealed to the world a pair of flesh-tone, control-top pants. Somewhere in LA there is an old lady with a draughty undercarriage, because these were granny's finest. On the other hand, if you were married to Tom Cruise, you'd be wearing a reinforced gusset, too.

Underwear really became a trend, though, on the day we were treated to G-strings on ice. Duncan James, the piercing-eyed hunk from the boy band Blue, is the latest celebrity to have been wounded while rehearsing for Dancing on Ice. But while his colleagues have sustained broken ribs and grazed chins, Duncan's is a more intimate injury. "It really is demanding on the groin area," he confessed. "The jockstrap chafes like hell. It is skin-coloured and cuts right up my bottom. I don't know how women cope with thongs. This show is the hardest thing I've ever done."

Duncan's misapprehension that women "cope" with thongs is charming. If he worked in an office rather than a recording studio full of boxer-wearing boys, he would have observed the continual half-buttock shuffle that marks distracted ladies peeling and pinging their repressive undergarments. Duncan has led a very sheltered life if he thinks that thongs are as bad as it gets. He should try cramming himself into Trinny and Susannah's Magic Knickers, a medieval device that makes one resemble a tube of toothpaste that has been squeezed in the middle.

But woe betide anybody who ignores the new full-coverage fashion. Poor Daniel Radcliffe has spent months preparing for his West End role in Equus, buffing his weedy wizard's body until Hermione might say, "Oh, but Mr Potter, you look [gulp] lovely without your glasses." But what does he get for his full-frontal loveliness? Parents writing to websites to say they are "very disappointed and will avoid the future movies he makes". The ingratitude.

Only one woman is brave enough to reject the gratuitous knicker scene, and she is being sued for it. Dame Kiri Te Kanawa has pulled out of concerts with the Aussie rocker John Farnham, for fear that people might throw pants on stage. It would be "absolutely horrendous", she said. Too right: there is only one situation in which an audience should be permitted to pelt a lady with underwear while she is performing. That's if she is Britney Spears.