When stroke victim Jean-Dominique Bauby's book was turned into a Bafta-winning film, the world wept for his tragic on-screen wife. But why was his lover, Florence Ben Sadoun, airbrushed from the story? She tells Janine di Giovanni a different version of events

Wrapped up in several sweaters against the chill of a wintry day in Paris, Florence Ben Sadoun sits in a bohemian tea shop near the Luxembourg Gardens. In front of her is a pot of strong coffee and several notebooks scrawled with her work. 'Catherine Deneuve helped set this up,' she smiles, motioning towards the comfortable sofas. 'It's fun. I can work here, peacefully.'

Abruptly the peace is broken when a teenage band begins to warm up, attempting - badly - to play scratchy French folk music. Ben Sadoun, who has an open face, with dark eyes, covers her ears, cringes, apologises for the noise, and sinks back into the cushions, wrapping a shawl around herself. She looks smaller than she really is, and vulnerable.

Seeing Ben Sadoun, 48, bundled up and shivering, reminds me of a scene in The Diving Bell and the Butterfly, the film based on the best-selling book of the same title, by her late lover, Jean-Dominique Bauby (Jean-Do as his loved ones called him). On 8 December 1995, a crippling stroke had left Bauby with locked-in syndrome - a rare neurological condition in which his mind was clear and alert but he could not move a muscle except for his left eyelid. He was transferred to a hospital on the windswept coast of northern France. In the film one of the nurses swathes the former editor-in-chief of French Elle in blankets and scarves and takes him in his wheelchair to look out towards sea. The power of the wind pushes the paralysed man back inside himself, to memory, the only part of his life that he can still control.

Florence Ben Sadoun also has memories, many of them intensely painful, many of them joyous. She was Bauby's lover and companion for three years, the woman who sat by his side, who held his hand when he died, who was planning on taking a trip with him even while he lay wasted in a hospital bed. Two or three times a week, she would drive the three hours from Paris to sit with him, read to him, joke with him. She, along with a handful of his closest friends, was part of what Bauby called 'my personal bodyguard'. She shared his days, which were spent spelling out his book with the help of assistants. Using a method known as the Silent Alphabet - which involved blinking his eyelid to depict the letters of words, one by one - it took him 200,000 blinks to complete it. He died shortly after it was published, with no hint that it would become an international best seller.

Despite Bauby's decline (one of his friends joked that up and down Boulevard St Germain, the fashionable Parisian set was horrified to hear that he was a vegetable. 'What kind of vegetable?' Bauby asked, not losing his sense of humour) the couple still shared affection and Ben Sadoun even endured his disapproval if he did not like what she was wearing or how she had done her make-up.

At the time of his stroke, they had already been together for two years. She remained loyal to him till the day he died. 'Florence, always Florence, at your side,' wrote Bauby's colleague, the editor of Elle, Valerie Toranian. 'The woman you loved, who loved you until your last breath.'

'He was the man I loved,' Ben Sadoun said at the time of his death. 'I never saw him as anything else. We stayed in love. We even had fights. It's what kept us both alive.'

And yet in the film version of the book, it is Bauby's erstwhile partner of 10 years, Sylvie de la Rouchefoucauld (they never married but had two children together - in the book, he refers to her coldly as 'the mother of my two children') who was given the role of the luminous Madonna. This despite the fact that she rarely visited Bauby in hospital and was in New York with a new boyfriend when he died in 1997.

In the movie, directed by the artist Julian Schnabel and written by the celebrated screenwriter Ronald Harwood, she is called Céline, and played by the beautiful Emmanuelle Seigner. She is portrayed as faithful, kind, patient. She is also sad and jealous - Bauby had already left her at the time of the stroke - but the film manipulates the audience so that Céline is pitied and adored by audiences for her beatific role. So much so, that as I was leaving the cinema where I first saw it, a friend of mine - who did not know the real version - said to me, 'That poor wife!'

During one memorable scene, Céline translates a loving message from Bauby to Florence (who is called Inès in the film) who, we're told, never came to see him because she couldn't bear his pitiful physical state. 'Each day I wait for you,' he signalled, using the Silent Alphabet. You see the agony of a jealous Céline holding the telephone to her shrunken ex; and you hear the sobbing voice of Inès. Later you see the shrivelled wife, standing on a freezing train track, waiting to go back to Paris, sobbing.

You feel Céline's jealousy, her anger, and her sense of injustice. You take her side. What a bitch this mistress must have been! Not only does she steal the husband but also then she does not come to see him in the hospital because she cannot bear to see his wasted body.

Except it never happened. That phone conversation never happened because Ben Sadoun was at Bauby's bedside constantly.

Lots of other details were changed in the film version for no apparent reason. They range from the ludicrous to the more insidious. Julian Schnabel gave Bauby three children in the film rather than his two, Theophile (11 at the time of his death) and Céleste (nine). Because he could not decide between the three adorable child actors, he cast them all.

'Jean-Do never said, "I want to die" to one of the nurses,' Ben Sadoun says with a touch of anger. 'He never said that. He said, "My mouth is full of chestnuts".' A black actor who brought Bauby a silly hat was portrayed as another close friend. 'He was not black in real life. He never brought Jean-Do a hat.' But it's the portrayal of Ben Sadoun as a weak-willed, selfish girlfriend, unable to face her once handsome boyfriend, which is the most striking example of 'artistic licence'. At one point, according to Ben Sadoun, Schnabel's wife, Olatz (who plays a beautiful physiotherapist in the film) pleaded with her husband to listen to Florence and tell the real story. He brushed her off, telling her to leave him to the directing of the film.

It is this cinematic version of the story, released two years ago to great acclaim, that has now become the accepted, official version of Bauby's life. Only a handful of friends in Paris know the truth about Ben Sadoun who, despite the unfavourable way in which she is portrayed in the film, had always chosen not to speak out about it. But now she has published her own account of events, ambiguously titled La Fausse Veuve (The False Widow).

What really happened is this: As Ben Sadoun tells it, Jean-Dominique Bauby was a charismatic charmer, a talented journalist and a mover and shaker on the Paris scene in the Eighties and Nineties. He loved fast cars, good food, and reading what he thought of as the hilarious English tabloids. Ben Sadoun was a journalist specialising in film, fashion and beauty. She had two children from a previous marriage. The two met at Elle magazine in the early 1990s when Bauby was editor-in-chief and Ben Sadoun was a critic.

They were close, but did not become lovers for a year, until 1993. 'We resisted,' she says. 'It took time.' This was complicated by Bauby's relationship with Sylvie de la Rouchefoucauld, a PR dynamo from one of the most famous families in France. The couple lived, with Céleste and Theophile, in a magnificent house outside Paris with a pool and tennis courts. By all accounts, Bauby liked the high life.

'Obviously it was tough, really tough, once the relationship began,' Ben Sadoun says, talking of their affair once it began. She says she hated the lying which comes with being a mistress. 'Holidays, horrible!' she shudders. De la Rouchefoucauld did not appear to know - although like many French women she probably realised that her husband was straying and decided the best tactic was to keep quiet. She never mentioned Ben Sadoun. 'Our relationship,' says Ben Sadoun quietly, 'Bauby and me, was very strong.'

It could have been a fling, but Bauby fell in love with Ben Sadoun, whom he described as a 'tall, beautiful woman with dark hair' and he left his family home for her. The two had separate apartments in Paris.

In the book, it is Ben Sadoun and not de la Rouchefoucauld who is referred to tenderly by Bauby on his last day before the stroke - 'Florence softly stroked the nape of my neck'. In fact de la Rouchefoucauld is hardly mentioned except for a Father's Day outing to the beach. Bauby had long fallen out of love with the mother of his children.

Bauby's friends, who came out in defence of Ben Sadoun after the film came out, were adamant that his love for her was strong. 'In fact, the only acknowledgement [Schnabel] gives me in the film is a wave at the window,' Ben Sadoun says sadly.

In France, publishers own the book and film rights, but after Bauby's death, the rights went to his children. De la Rouchefoucauld was consulted in the making of the film, much to the chagrin of his inner circle of friends, including the photographer Brice Agnelli and writer Bernard Chapier. They knew the truth and were appalled at the way Ben Sadoun was portrayed.

'It is not the story of my friend,' Agnelli said after seeing the film. 'It is a story for Hollywood.'

It must have been intensely painful for Ben Sadoun to see her real-life love story, which was so poignant and so loving, turned into a cinematic mess of lies. Furthermore, the film, made by a painter turned director, is stunningly beautiful: full of greens, blues, and the colours of the sea. The sets are breathtaking. It was nominated for Oscars, won a Golden Globe and a Bafta. Bauby was played by Mathieu Amalric ('In real life he looked nothing like Jean-Do,' Ben Sadoun smiles. 'But in the film! Exactly! It was him!'). And it was widely acclaimed as a work of genius. For a film critic like Ben Sadoun, it must have been unbearable. (Valerie Toranian, the editor of Elle, was so offended by how Ben Sadoun was portrayed that she did not allow filming at their offices and the magazine did not officially review the film).

Two years on, Ben Sadoun is now happily married to an actor whom she does not want to discuss. 'It was hard for him, too, going through all this,' she says. 'In a sense, we want to put it behind us.' Mainly, she says, she does not want to feel bitter. 'I loved Jean-Do, even when he was in the hospital, even when he could barely move an eye. I still loved him.'

She has written La Fausse Veuve not as an act of revenge, she says, but because friends of Bauby were so annoyed at the inaccuracy that they encouraged her. The result is not a memoir. Nor is it a novel. It is something in between - 'Auto fiction, quite common in France'. There are no names to the lead characters, no indications that it is a true story - unless you know what really happened. 'It took a long time to write it,' she says. 'It's not a witness account... it's not really my version, years don't match... but I took the memories.' It is, she says a 'mélange of history'.

After Bauby 's death in 1997, Ben Sadoun took to her bed for three months. 'I could do nothing. I slept,' she says. 'I could not work, read, do anything. Friends took care of me.' In a sense, she had grown used to her lover, even in his sad state, and could not imagine a life without him.

'Brice, Florence and I had to cling to each other,' says Bernard Chapius. 'Otherwise, we would have died.'

But gradually, she revived. She raised her children, went to work for another film magazine - immersed herself in her writing. Eventually, she met another man and finally she began to put pen to paper about Bauby. When she saw the film, she was startled, upset, 'disgusted', she says. But she made a decision not to let it colour her memories. Instead she has turned the sadness into a beautiful book, full of longing, life and love. And, above all, dignity.

Who, I ask, is the fausse veuve? Is it de la Rouchefoucauld, portrayed as the grieving widow in Schnabel's film, or is it Ben Sadoun, who never got to be the widow, but was, in effect, the real one?

'It's me,' she says, smiling broadly. It is an unusual love story, but an exceptionally beautiful one, and most importantly - it is true.

• La Fausse Veuve is published by Denoël Editions