Beyoncé Knowles in the movie Dreamgirls fronts a trio of black, bouffanted teenage girls who wiggle and sing in matching red spangly dresses — America’s sweethearts, a group who racked up an astonishing 12 No 1 hits in the US. We know them at once and we know those hits. Except she isn’t Diana Ross, she’s Deena Jones. The song isn’t Baby Love, it’s something called Family. And she isn’t fronting the Supremes, her group is called the Dreams.

The fiction of Dreamgirls, which opens next month, is so close to the story of the Supremes that Beyoncé has even spoken to Diana Ross and asked for her blessing: “All those strong black women, that’s what I strive to be,” she told The Times in December. The problem is that in fictionalising the story of Ross, the Supremes, and the early days of Tamla Motown, Deena Jones may come over as rather more strong and black than the public perception of Diana Ross.

The look of Dreamgirls is a little too powdered and a little too fancy dress, as is often the case with films based on semi-fictional Sixties groups. It looks a little silly. The real thing can be found on The Supremes: Reflections, a DVD anthology released just before Christmas. Here we see Ross, always happy to grin and fawn, too eager, far too eager to please. All angles, bones, and teeth, an ever-ready smile. But not Florence Ballard, the group’s founder, original lead singer, the Supreme who came up with the name. She twirls and smiles behind Ross with Mary Wilson, but knows it’s always pop, always fun, first and foremost.

Flo chewed gum, talked back, had dangerous curves. Too dangerous for the Motown boss, Berry Gordy, who was carrying on with Diana. Stick-thin Diana was still called Diane by her mum and dad: she added the “a” to give herself class. And Flo was eventually evicted from her own group — in 1967, at the height of their powers — by Gordy, who believed she had weight problems. Her last appearance on the Reflections DVD, on Love is Here and now You’re Gone, is poignant and shows her to be, even in these cake-conscious times, curvy and beautiful and a perfect fit in her red spangly dress.

The virtual Flo, Effie White, gets a showstopping number to herself in Dreamgirls called And I amTelling You I’m not Going, which is quite as pompous and bodice-busting as the title suggests. In the movie it’s the bomb, a solo smash that launches her new career. In reality Ballard’s first single was a minor confection called Love Ain’t Love that sold badly, but not as badly as the handful of 45s that followed it as Flo swiftly became a former pop star. Four years later she was on welfare, drinking hard. Eight years later she died.

White doesn’t die in Dreamgirls. No one benefits when history is twisted in this way. Tampering with the Supremes story clearly does nothing for the memory of Florence Ballard. Yet it also strengthens the hand of Ross’s many detractors who see her as manipulative, a schemer and, worse, not a real “soul” singer. Not black enough.

Dionne Warwick has a sister, Dee Dee, whose voice can scorch earth and shatter glass: the very definition of soulful. Dee Dee had minor hits but Dionne became Burt Bacharach’s muse, because her technically weaker voice had a catch, a unique quality that her embittered sister’s lacked. Likewise, Ballard and Wilson could both sing Ross under the table and out of the window, but Gordy was right: Ross was the star. Not as cute, as sexy, or as intense, but her voice was as sweet and digestible as vanilla ice cream and as recognisable as Jagger’s or Lennon’s. She was the biggest single influence on Michael Jackson, one of the biggest influences on a generation or three of American soul singers.

Ross could set the record straight, confess that she’d get her chauffeur to cruise around the Brewster projects of Detroit to remind her of who she once was and how far she’d come. Flawed perfection — people really wouldn’t mind. Instead, she has form. In 1973 Ross played Billie Holliday in the 1973 biopic Lady Sings the Blues. Funnily enough it evaded tales of pre-teen prostitution, and changed several aspects of Holliday’s life in order to make the film more unified and dramatic. And it won Ross a Golden Globe.

For Florence Ballard’s funeral, a crowd of 5,000 lined the streets of Detroit. Some booed when Ross appeared and, nervelessly, took a place alongside Flo’s immediate family. The Detroit Free Press noted that “Ms Ross’s eyes filled with tears, and the photographers snapped her picture”. When they took the casket out, the organist played Someday We’ll be Together, the Supremes’ last No 1, and a record on which they were billed as Diana Ross and the Supremes. Ballard had left the group two years before Ross recorded it with a few session singers.

“The elements of determination and strength in her personality that were chiefly responsible for her successes are lauded when displayed by a man,” says Thomas Adrahtas in a new, overwhelmingly positive biography, Lifetime to Get Here: Diana Ross, the American Dreamgirl. His feeling is that she isn’t getting her historical due.

A few weeks before her new album, I Love You, is released, Diana Ross’s story is still being bent and disfigured; truth, myth and lies now inseparable. Reviewers anticipate a mauling. The irony probably is lost on Miss Ross, but in 30 years no one has printed a bad word about Florence Ballard.

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