We used to adore the famous. Now we prefer to revel in their shortcomings, from wobbly thighs to lack of brainpower. Nadine O'Regan asks what has prompted our change of heart

They are stars, but suddenly you barely recognise them. On one magazine cover Cameron Diaz sports a face full of nasty red spots. On another Martine McCutcheon reveals wobbly thighs and arms. Lara Flynn Boyle looks as if she might be anorexic. Britney Spears is shown bursting out of her tiny tank top.

Many of the images are discernibly candid shots, taken when the stars were unaware a paparazzo was anywhere near them. But even in officially sanctioned photographs, in which they are gracing a red carpet and smiling for the cameras, decked out in their finest borrowed Versace, a negative spin is often put on the images.

"It's hard to believe this is the same Queen of Pop who's on a sell-out world tour," one magazine pundit sniggered about Madonna recently. "Turning up in a mumsy slip is not very rock 'n' roll." Donatella Versace faced even more withering scorn. "As a world-renowned designer, you'd think Donatella would know what suits her by now." Cheap jibes, you might think, but you can see why the journalists would feel compelled to make them.

Celebrity bashing has become the latest sport to enthral the magazine-reading public. The celebrities we once loved to adore we now appear just as happy to mock. Spears is said to have more hate websites than Saddam Hussein, and so-called water-cooler debates revolve around how much cosmetic surgery the stars have had - and how atrocious they looked beforehand.

"When we put one of our surgery features on the cover the circulation blew up," says Russ O'Connell, Heat magazine's deputy picture editor, cheerfully. "A lot of people had seen these celebrities without knowing that they had had surgery. There's a great interest in this, and the circulation has reflected that."

Demand is also high for photographs of celebrities in unexpected circumstances. When Kevin Mazur took the first secret shots of Brad Pitt and Jennifer Aniston together - Pitt had his arm around Aniston at a 1998 Radiohead concert in Washington DC - he made a staggering €75,000 from sales of the images.

Photographs of celebrities looking ugly also regularly fetch premium prices. "There's more demand for that kind of shot," explains Phil Loftus, founder of the Capital Pictures agency in London and a celebrity photographer for 23 years. "Anyone can get a nice picture of a celebrity, because that's how they present themselves. But if you can get a picture of a celebrity looking grotesque, fat or badly dressed the picture is rarer. These are the pictures the public want."

Why they should want them is easy to figure out. "Celebrities had been deemed untouchable," says O'Connell. "But now people have realised that they're the same as everyone else. They do get spots. They do get sweat patches. They have bad hair days like everyone else. People want to see that they are just like us. They are normal."

Back in the halcyon days of Hollywood, publicists did everything possible to prevent this perception of stars' normality forming in the minds of the public. Famous actors had to maintain their aura of glamour and mystique at all costs.

But their carefully constructed facades are impossible to emulate now. The creation of the Internet, the explosion of the celebrity-magazine market and the rise of multichannel television have contrived to create an environment in which flaws cannot be concealed.

In attempting to make the best of the situation, many celebrities have revealed hitherto unknown entrepreneurial talents. "It's a bit of a secret," says Loftus. "But a lot of B-list celebrities collaborate with photographers on paparazzi pictures. The photographer takes what are supposedly paparazzi shots and shares the proceeds from the magazine with the celebrities."

The ruse doesn't always work. When the television presenter Anne Robinson was snapped by a paparazzo jogging near her home in the Cotswolds she tried to make a deal. If he ditched his original shots of her looking sweaty and ugly he could have an exclusive the following morning of her jogging about the place looking chic and groomed. The photographer agreed.

It was only when Robinson opened the Daily Mail that she discovered she'd been double-crossed. Both sets of photographs had been published in an embarrassing montage, with a sneering editorial attached.

Robinson was likely steaming with rage, but at least she didn't have to contend with having a photograph of herself spliced in to an electron micrograph of the Ebola virus. This has been the fate of Mutya Buena, a member of the British pop group Sugababes.

The bleakly humorous Blamemutya site suggests, with appropriate disclaimers attached, that Mutya is deeply nefarious. "Mutya Buena is the evil Sugababe," the commentary runs. "But just how evil is she? What unspeakable evil acts can be blamed on the evil one?"

Doctored images show Buena looking shifty in the crowds near John F. Kennedy's cavalcade, standing beside President Bush and even striding along happily with Hitler. You would guess Buena would not be overly pleased. But in a recent interview she seemed flattered. "One day I was a nobody," she chirruped. "Now I have my own website."

Many other celebrities have had similarly positive reactions to their public annihilations and humiliations. It was widely predicted that Jade Goody, a Big Brother contestant in 2002, would fall apart once she read what tabloid reporters had been writing about her. Instead she laughed it off and flourished.

The hotel heiress Paris Hilton has blithely promoted her television career in the wake of an infamous video of her having sex with an ex-boyfriend, which surfaced on the Internet last year. And Jessica Simpson, the American singer and reality-television star who wasn't sure whether tuna was chicken or fish, has cheerfully agreed with reporters that she would not exactly rival Einstein in the brainpower department.

The stars' lack of animosity towards the media depiction of their public embarrassments is easily explained. Sure, they've been mocked, and sometimes even reviled, but in the process they've got exactly what they wanted: fame and money. In fact, the worse they appear the more fascinated we become with them. Before her MTV show Newlyweds and the tuna incident aired, Simpson was just another starlet drifting in to pop oblivion. Now her album is outselling both Britney and Beyoncé in the US.

Simon Cowell, the Pop Idol judge, has promoted his brand of nastiness so effectively that he is known to millions stateside.

As for the linguistically challenged former pop star Peter Andre, he's back in the charts after resurrecting his career with a stint in a jungle and a problem with easy vocabulary.

So the situation seems clear. We want no more glamour or mystery. No more elusiveness or hauteur. Instead we, the viewing public, want the humdrum: the stupidity, the spots, the fights and the flaws. The kind of stuff you'd see every day, in other words. This is the new celebrity. As Andre might say, it's insania.