Six years after he died, the 'truth' over Yves Saint Laurent's hedonistic and tortured life is being fought over by two new films

In 2001, seven years before he died, Yves Saint Laurent agreed to be filmed by documentary-maker David Teboul for a rare behind-the-scenes look at his work. In the opening scene, watching a slideshow of family photographs, he grimaces: "J'ai joué le 'grand couturier'…" His voice is both sad and self-mocking; the voice of an old man looking back across a great distance at his frail 16-year-old self, head bowed over his lavishly dressed paper dolls.

Growing up in 1940s French Algeria, the young Yves Henri Donat Mathieu-Saint-Laurent dreamed of Paris: a bullied outcast at school, he escaped into fantasy at home – devouring his mother's fashion magazines, sketching endlessly, and predicting (in the safety of his adoring family circle, at least) a future of spectacular fame.

Six decades on, the story of the little boy who played "grand couturier", and who grew up to become the century's most notorious fashion designer, shows no sign of losing its appeal. This year will see the release of two films based on his life: the first, actor/director Jalil Lespert's Yves Saint Laurent, is currently topping the French box office, charming audiences with its affectionately human portrait of the man behind the myth. In October the second – directed by Bertrand Bonello, best known for the controversial L'Apollonide, or House of Tolerance – will be released.

Pierre Bergé, Saint Laurent's partner, has granted Lespert access to his extensive archives and last year tweeted his outrage at the rival project: "I hold the moral rights over YSL's work… A trial on the cards?" Still, Bonello has his own heavyweight backer in Kering – the luxury conglomerate which now owns the Yves Saint Laurent label. His biopic will focus on Yves during the years when he accomplished his definitive works (the decade ending his triumphant Ballet Russes collection in 1976); and Bonello is playing up his version as the "unauthorised" story – one which will portray Yves's truth, rather than Pierre's.

It is the latest round in the battle for the designer's legacy. Since the late 90s some of the biggest names in fashion, from Lanvin's Alber Elbaz to Tom Ford, have spent periods at the label's helm; ultimately even the all-conquering Ford was defeated – simply, it seemed, not Yves enough. Saint Laurent himself died of brain cancer in 2008; four years later Hedi Slimane – first hired as a menswear designer back in 1996 – returned to the house and immediately shortened the label's name to Saint Laurent. The move provoked an immediate storm of protest, and a surprisingly long-lived backlash: last autumn, Parisian store Colette fell out with the label over T-shirts which bore the slogan: AIN'T LAURENT WITHOUT YVES.

It's not hard to understand the fascination with the YSL story, a saga that blends wild public success with private suffering. Outwardly Saint Laurent led a charmed life. By the age of 18, he had won an international design competition (beating future rival Karl Lagerfeld) and been hired by fashion's reigning Sun King, Christian Dior. Lespert's film begins shortly after, with Yves enthroned as Dior's crown prince. As played by Pierre Niney, he is at once violently demanding and intensely shy, pampered and secure – as he had been in Algeria – in a world of adoring women. But that sheltered, happy existence was soon to be shattered, with Dior's premature death in 1957 pushing his seemingly reluctant successor into the spotlight.

Into the limelight: Yves Saint Laurent with models in 1962. Photograph: AFP

Saint Laurent became an object of immediate fascination: quiet, timid, with neatly parted schoolboy hair, anxious eyes lurking behind thick glasses and a frail body encased in a tight black suit. L'Express hailed him as France's latest enfant triste – another of the country's new wave of melancholy prodigies, like novelist Françoise Sagan and painter Bertrand Buffet. Womens Wear Daily's correspondent was more specific, and less charitable – she saw "an ugly, ungainly, overgrown boy with thick glasses, and so horribly shy he couldn't take his eyes off the floor".

Charged, at 21, with safeguarding the future of the world's most successful fashion house, Saint Laurent began well: after his first, rapturously received collection, the International Herald Tribune's correspondent reported: "Everybody was crying. It was the emotional fashion binge of all time."

The rapturous reviews were soon followed by doubting ones, and two years later the House of Dior took advantage of Saint Laurent's military call-up to have their boy wonder replaced. Exiled from his Avenue Montaigne paradise, he fell apart and was admitted to mental hospital a bare three weeks after reporting for duty: a has-been at 24.

But Yves was not alone. Shortly after his Dior début in 1958, he had met Buffet's then-boyfriend, Pierre Bergé, and embarked on a personal and professional relationship which would endure to the end of his life. Bergé got Yves out of hospital and back to work, helping to set up the label whose three sensuously entwined initials would revolutionise Parisian fashion in the 60s, scandalise the world in the 70s and stamp themselves imperiously across the 80s.

From the outset, their roles were clear. Yves was the vulnerable, suffering artist and Pierre the fiercely controlling protector: a man who, in Lespert's film, is painfully aware of his public image – "the pimp who's found his all-star hooker". And in the frail Niney, the director has found an actor perhaps more like Saint Laurent than Saint Laurent himself ever was: tortured, intense, and yet endearingly childlike as he stumbles out of couture's disciplined universe into the liberated decadence of the 60s.

There are signs of a dark side, too; the jealous possessiveness of friends, the trembling fear of physical intimacy, the ability to work himself up into convenient hysterics at the slightest hint of pressure. And there are flashes of a strain of steel that few saw, or chose to see. "Yves was a very strong person," says Susan Train, Condé Nast's Parisian bureau chief and a friend of both Saint Laurent and Bergé. "But he was much happier to keep Pierre out in front, making all the noise and doing the barking and nipping at people's heels. Yves depended enormously on Pierre, and he would never have been the success he was without him."

That success took time: the first Yves Saint Laurent show, in January 1962, received a moderate response. But in the years that followed, his star soared. He created ideas that became overnight sensations and then timeless icons: the Mondrian-print shift dress, the Saharienne safari jacket, the Le Smoking trouser suit, Catherine Deneuve's Belle de Jour wardrobe. And in the mid-60s, his ready-to-wear Rive Gauche label became a global phenomenon, offering women an affordable slice of the YSL dream. The young, timid Yves had gone, replaced by a charming, seemingly assured man who was more than just a household name – like Coco Chanel, he had become his brand's most alluringly potent incarnation.

Magazines captured images of Yves at play: partying with Halston and Warhol in New York, making mischief with muses such as Betty Catroux and Loulou de la Falaise in Paris, and relaxing with the kif-smoking jet set in Marrakech. His dark suits disappeared, replaced by luxuriously louche kaftans, silk shirts, suede jackets and leather trenchcoats. And when he stepped in front of Jeanloup Sieff's lens to promote his first men's fragrance in 1971, he went nude. His instructions were specific: "I want to create a scandal."

The art of dressing: Le Smoking suit, 1967. Photograph: Reg Lancaster/Getty Images

That image became a desperately effective mask. The paparazzi-friendly Yves who danced the night away in the 60s and 70s was high – on success, on fame and on an ever-changing cocktail of alcohol, acid and cocaine. Off camera there were fierce, increasingly violent rows with Pierre, who was struggling to keep both the business and Yves himself afloat. Ultimately Bergé would move out, unable to cope with Yves's utter self-absorption. As the years went on they both had other interests, other passions, other lovers (most notably Lagerfeld protégé Jacques de Bascher, whose affair with Yves added another dimension to the bitter Lagerfeld/Saint Laurent rivalry).

They continued to function as a symbiotic double act to the end. And while Pierre became an increasingly belligerent spokesperson, Yves flinched away from the public gaze, exhausted by the fashion treadmill and yet apparently unable to stop. There were rumours of illness, of Aids, and regular premature reports of his death. In his diaries Warhol recorded: "Loulou [de la Falaise] told us that YSL really was such a genius that he just can't take it, he has to take a million pills and the whole office gets so depressed when he's depressed…"

Finally Yves simply withdrew: Pierre commented that his partner had "entered depression as one enters a religion". And, as ever, Bergé took care of everything. "Everything I didn't have, he had," Yves said in 2001. "His strength meant I could rest on him when I was out of breath."

Even at his most extroverted moments, Yves had been shielded by his cabal of intimates; towards the end, his world was reduced to his studio on Avenue Marceau, the couple's holiday home in Marrakech and the cloistered apartment on Rue de Babylone to which fewer and fewer people were admitted. At the same time, he signed his name to everything from sunglasses to bed linens to cigarettes – and licensed a range of era-defining fragrances (Opium, Jazz, Kouros), which would keep YSL's name firmly in the spotlight as the man behind the initials slowly faded away.

If the two new films are likely to reveal anything it is this: the man himself remains an enigma. Early in Lespert's film, he confides in Bergé: "You know, I'm not that nice." And you sense that not-niceness remains the largely untold part of the Saint Laurent story – the man who could cut friends and supporters out of his life without a backward glance, who averted his gaze from unpleasantness and who sheltered behind Bergé's aggressive energy just as completely as he sheltered behind his thick-lensed glasses.

Pierre Niney and Charlotte Lebon in the new film Yves Saint Laurent Photograph: Thibault Grabherr

Yves was a genius, indulged and excused by a generation that believed that geniuses should live by different rules. In some respects that universe still endures in fashion, particularly in Paris: it's only been three years, after all, since John Galliano – another indulged Dior boy wonder – had his own startlingly public fall from grace.

Saint Laurent continued designing until 2002 – every show remorselessly measured against his past hits, and every final bow accompanied by the suspense of waiting to see whether he'd manage the short walk to the end of the runway. Lespert's film ends with Yves stumbling on to the runway, mouth slumped askew, eyes lost behind his glasses, his movement unsteady and uncertain.

Which of the films will be closer to the 'truth'? It is hard to say. Even in life, Yves Saint Laurent spent decades lost behind a screen of conjecture and rumour: his story has long been more legend than fact.

Yves Saint Laurent is released on 21 March