But now "Belle de Jour" – whose diary about her secret life as a prostitute inspired a television series starring Billie Piper – has revealed who she really is.
Dr Brook Magnanti, a research scientist from Bristol, yesterday revealed herself to be the secretive author of the Diary of a London Call Girl, which has drawn millions of readers to the internet for the best part of a decade and spawned a series of imitators.
Dr Magnanti kept her secret from her own agent, receiving royalties through a roundabout money trail, and also from her parents until this weekend.
She revealed who she was to a London newspaper columnist who had accused her of glamorising prostitution.
Dr Magnanti, 34, is a specialist in neurotoxicology and cancer epidemiology with a Bristol hospital research group. She said that she spent 14 months as a prostitute in London, working with an escort agency, after she ran out of money during her studies.
Writing anonymously, however successful, was "no fun" and the secrecy was making her paranoid. "I couldn't even go to my own book launch party," she said.
But she also hinted that her hand may have been forced by someone else threatening to tell her secret, with "an ex-boyfriend with a big mouth lurking in the background".
She told workmates at the Bristol Initiative for Research of Child Health of her secret life about a month ago. They were "amazingly kind and supportive", she said.
Belle de Jour's blog became a hit on the internet in 2003. Her name – meaning "beauty of the day" – was the title of a 1920s novel and 1967 film starring Catherine Deneuve.
She became the author of three books and a newspaper column, while Doctor Who actress Billie Piper starred in the television adaptation.
Meanwhile, journalists and critics debated whether the blog was true, or perhaps the anonymous work of an established writer, or even a man.
Last month the Archbishop of York singled out Belle de Jour in a speech attacking the glamorisation of prostitution. The identity of another anonymous sex blogger, Girl With a One Track Mind, was exposed three days after her book was published.
From late 2003 to 2004, Dr Magnanti said, she worked as a prostitute through a London escort agency, charging 300 an hour for her services and receiving 200. She had slept with "between dozens and hundreds" of men, she said.
Studying for her PhD she had run out of money for rent, and needed "cash in hand" without training or investment. "I didn't think I'd be doing myself harm," she said. "I didn't go into it cheerfully, but I wasn't resigned and broken either. I've felt worse about my writing than I ever have about sex for money."
'IT FEELS SO MUCH BETTER NOW…'
BELLE de Jour, the diary of a London call-girl, first appeared on the internet in October 2003. The first entry described her as a part-time prostitute.
"Located what sounded like an excellent small, discreet agency (word of mouth, as they say). After e-mail contact and sending my photos, I finally arranged to meet the manager."
Later topics ranged from her taste in Jimmy Choo shoes or Victoria Beckham-designed dresses to her on-off boyfriend and handling difficult clients, or chatting to them over the finer points of arthouse cinema.
"Regarding orgasms at work," she wrote. "I don't… Let's be honest, this is a customer service position, not a self-fulfilment odyssey…"
Yesterday, after revealing herself to be Dr Brooke Magnanti, Belle wrote: "Now I'm not anonymous. Looking back over my diaries is sometimes embarrassing, sometimes hilarious (often unintentionally so). Belle and the person who wrote her had been apart too long. I had to bring them back together. It feels so much better on this side …
"I am a woman. I lived in London. I was a call girl."