"We might need to see you without your bra on."

A beautiful woman sits in front of a video camera. Her name is Sena Cech and she is a fashion model. Her tone is matter-of-fact, as though what she's about to describe is commonplace in the industry in which she works. The scene: a casting with a photographer, one of the top names in his profession. Halfway through the meeting Cech is asked to strip. She does as instructed and takes off her clothes. Then the photographer starts undressing as well. "Baby - can you do something a little sexy," he tells her. The photographer's assistant, who is watching, eggs her on. What's supposed to be the casting for a high-end fashion shoot turns into something more like an audition for a top-shelf magazine. The famous photographer demands to be touched sexually. "Sena - can you grab his cock and twist it real hard," his assistant tells her. "He likes it when you squeeze it real hard and twist it."

"I did it," she shrugs, looking into the video camera. "But later I didn't feel good about it." The following day she hears that the job is hers if she wants it. She turns it down. "I didn't like the way the casting had gone. If the casting was that sexual I was sure the job would be really sexual and gross." The photographer never offered her work again.'

This is the ugly, sleazy side of the modelling industry, the side few insiders like to talk about. It's one of the most secretive businesses in the world, which is ironic when you consider that it is also one of the most pervasive. Its stars are some of the most recognised icons of our time, household names whose bodies are frequently emblazoned across 40ft-high billboards, yet apart from the occasional flurry of publicity about anorexia or drug-taking, outsiders know surprisingly little about the multimillion-pound business which profits from some of world's most beautiful women. Models rarely give interviews, and if they do they're as studiedly anodyne and vague as Premiership footballers quizzed outside the changing room after a match.

Sena Cech is one of a handful of models who has decided to talk publicly about the seedy, unglamorous and, on occasion, abusive side to her profession for a new documentary, Picture Me. The woman behind the film is Sara Ziff, a catwalk model turned documentary maker.

Ziff makes an unusual whistleblower. She's made hundreds of thousands of dollars from the modelling business. Her motivation for speaking out has nothing to do with revenge or failure (when I ask her what it's like to be rejected for a job because of the way you look, it's clear this has not happened to her very often). She's been the face of brands like Calvin Klein, Tommy Hilfiger, Stella McCartney, Dolce & Gabbana and Gap. Her long limbs and angular cheekbones, almond-shaped blue eyes and blonde hair have adorned hoardings in Times Square and beyond. She's strutted down the catwalk, eyes blank, unsmiling, for all the top designers from Marc Jacobs to Louis Vuitton, Gucci to Chanel.

Picture Me began as a quirky homespun video diary. Ziff's former boyfriend and co-director Ole Schell would often accompany her on jobs, and because he was a film-school graduate it seemed natural to take along the camera equipment in order to make sense of the surreal, insular world in which they found themselves. The earlier parts of the film reflect the lighter side of the industry such as the camaraderie among the models and the buzz of a catwalk show. Schell would also document their private moments: arguments about money because Ziff was earning Monopoly amounts and he could not compete; Ziff in the bath after a long day at a shoot.

The process might simply have highlighted an industry as fake and frothy as a bowl of Angel Delight, but what emerged over the course of five years of filming and hundreds of hours of footage was something darker, more subversive. They started giving the camera to fellow models, putting them on the other side of the lens and giving them a chance to speak. Gradually the couple became less like innocent home-movie makers and more like undercover reporters.