Her sexual frankness and seeming inability to be shocked by anything have attracted male and female devotees alike to her weblog, on which she casts a critical eye over the people who pay for her services.

But her rise to literary fame, through a substantial book deal, is in danger of being brought to an abrupt halt. There is growing evidence that Belle may be a fictional character who has never been a prostitute.

A detailed analysis of her writing by the man who famously unmasked Joe Klein as the author of Primary Colors has convinced him that she is Sarah Champion, a 33-year-old author from Manchester.

Don Foster, America’s foremost literary sleuth, identified quirks in Belle’s text, such as the way she uses brackets, dashes, compound verbs and italics. He entered this information into Google, the internet search engine, and within 20 minutes found that Miss Champion was the only person who matched the linguistic fingerprint.

“While no piece of evidence is conclusive by itself, I’m sure we have found our woman,” he said.

Belle’s agent was anxious last night to deny that Professor Foster had identified the correct person. Patrick Walsh, who faces the collapse of the five-figure book deal if Belle’s true identity is confirmed, insisted: “I have never heard of her.”

Ms Champion bears an uncanny resemblance to Belle, although the website call girl is meant to be nearly ten years her junior. They both display an extraordinary love of obscure bands, have relatives in Yorkshire, have spent time in Manchester and are widely read in modern literature. Belle shows a detailed knowledge of South London and describes how she was “divided in love (from her boyfriend) by the A23”.

In 2002 Ms Champion was living in West Norwood, South London, less than a mile from the trunk road. She grew up in Manchester but left school before doing her A levels to become a freelance journalist, writing for the New Musical Express and the Manchester Evening News.

She briefly found recognition as a writer in the mid-1990s, editing anthologies about alternative culture, including And God Created Manchester and Disco Biscuits. She has been described as “a former publicist and private detective and today a global party correspondent and the compiler of techno and drum ‘n’ bass CDs”. She left Britain in 2000 for Bangkok, returning in 2002.

Last night Ms Champion, who is currently living with her boyfriend in California, did not deny that she was the woman behind Belle. “I’m in San Francisco at the moment . . . to hide,” she said, laughing.

She said that she was aware of the impact Belle de Jour had had and asked: “How did you find my name? Give me a clue?” Later she sent an e-mail with the statement: “I want to make clear that I have never been a call girl.”

Belle posted a message on her website yesterday afternoon, denying that anyone had come close to identifying her. “The people who have been ‘outed’ as me are not me, and to those for whom it attracted unwanted attention, I apologise,” she wrote. “And to those for whom the attention is wanted, are you mad? “Still, if you think the mantle of BdJ such an appealing guise, do let us know and I’ll have a pair of powder-blue Ginas spirited over to you forthwith. Or you could confess my sins.”

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