Yes, Belle du Jour was a prostitute

Belle was initially mistrustful - "so my boundless excitement did seem a little onesided. But things warmed up and she responded very well to the humour of the piece, which I think meant a lot to her, and she queried and pummelled us on our sexual realism; making sure we are aware exactly what goes on between consenting adults and not to be coy about it."

In the drama, Belle's day job is kept a secret from family and friends who think she is a legal secretary. "I think it connects to the way we all compartmentalise our lives, whether it's work or family or an affair or any other subterfuge," says Prebble. "It does spill over into things we can all empathise with."

Of course, Belle is a middle-class fantasy; a witty, university-educated woman who loves sex. Today she has made so much money from her writing she doesn't need to work as an escort. Which is not the reality for most prostitutes in London, who do it because they have drug problems or families to support or have even been trafficked from Eastern Europe.

Prebble agrees the drama deals with a very small sliver of an industry. But she insists, "After speaking to so many women, admittedly almost entirely 'escorts' rather than streetwalking women or flat girls, I was struck by how removed it felt from the tales of trafficking and exploitation.

"Of course people will say it is all part of the same thing. The buying and the selling of women. The using of people. But I just cannot accept that. Opportunity is everything. Of course if somebody steals your passport, locks you in a room and forces you to have sex to pay back a fictional 'debt', then that is rape and that is slavery. But to say it is the same thing as a privileged woman taking hundreds of pounds for consensual intercourse diminishes that suffering."

She met female escorts by contacting them over the internet, and was astonished how similar many of them were to herself. If you met them in a wine bar, she tells me, you would never guess.

What fascinated her about Belle's blog was not the lingering descriptions of La Perla underwear, but her voice. Finally here was someone showing that women could be quite clinical, detached even about sex. "The way she speaks, her sense of humour, the self-awareness, even some of the arrogance and coldness of what she's saying resonated with me."

Prebble was also attracted by the contradictions inherent in Belle's situation: "I wanted to do something with a very strong female protagonist with a moral ambiguity."

As a playwright, Prebble has a hugely original voice. The Sugar Syndrome, which portrayed the relationship between a bulimic teenager and a 38-year-old gay paedophile, showed an astonishing assurance for a 22-year-old. It was un-PC, often very funny, with a real generosity about people's damaged lives. But it was also highly moral.

"In my generation it is considered very rude to have a moral opinion on what someone else is doing which I think is a very dangerous thing. It's that live-and-let-live celebration of selfawareness over good behaviour and over kindness."

The play won the Critics' Circle Award and the George Devine Award for Most Promising Playwright, and was nominated for the Evening Standard Most Promising Playwright Award. What made the story even better for journalists was that, at the time, Prebble was actually working as a secretary at the National Theatre in Nick Hytner's office.

From the age of 14 she always had a parttime job, including stints as a secretary in a bus factory and as a beauty-salon receptionist, a fact she puts down to her parents. "I come from a background of people who go and work in the City, who completely accept that's how you make your money.

"My dad was orphaned. He grew up in a castle, below stairs, in Aberdeenshire, his father was a butler who died.

"Then he came down here and made all his money in software at the height of the computer boom. Business is regarded as a good thing in my family. And I've always had quite a struggle with that, growing up as a quite a spoiled liberal," she laughs.

As a child she always wrote plays and short stories. It didn't make her universally popular. She often retreated into her own world. But university was a revelation (she studied English at Sheffield). She toured plays to the Edinburgh Festival and began showing people her work. "I suppose I wanted to impress people but I also liked the control you get from writing. You are literally creating a world like a model railway track or a little town."

Her first short play, Liquid, about young City workers, won a student award. Then she got the job at the National, which she adored. "It was the first year Nick Hytner arrived and it was electrifying. It felt as close as you can get to working in The West Wing."

After the huge success of The Sugar Syndrome, she gave up the job. But it felt lonely. "I wasn't quite sure even if I wanted to be a writer," she recalls. "I had to go away and live a certain amount of life."

She wrote a play and several TV treatments that didn't get made. Part of the problem was finding a topic she felt passionate about. And, of course, people inundated her with ideas about young women and sex.

Secret Diary of a Call Girl was originally written for C4, but it bailed out. Perhaps bravely, ITV2 took it on and gave Piper her first grown-up role. The first episode, however, gives us a lot of Piper rolling around in her underwear and not a huge amount of dialogue.

Prebble acknowledges the limitations. "There are certainly things that if I had a chance to do a second series I would be insistent on doing ... but as it goes on and you get to know Billie's character better, you get a much greater depth and emotional truth."

As for the controversy, she has already emailed her family to warn them there will be bad press. "I said I'm totally expecting it, you may even agree with some of it, please don't get hurt on my account. But then I had lunch with mum and she said, 'Whatever is in the show, you do know we're very proud of you.' Which was not something that was said a lot, so that meant an awful lot."

When it comes to her writing, she is steely. "I think our utter inability to accept the very notion that a woman could charge for sex out of choice speaks to something terrified in our society. And when I am scared by something I choose to explore it."

Her next play is about corporate fraud - "a very male, quite white, middle-class place". But first there is Secret Diary of a Call Girl. She sincerely hopes she has created a great female anti-hero for mainstream TV. And she reveals the real-life Belle has been a great support. "She could very easily disown, distrust or ignore our drama; it would make no difference to her. And yet when we have been accused of various things by the press, she has come out swinging. I find that very generous and gratifying."

Secret Diary of a Call Girl starts on Thursday on ITV2.