The movie first made the headlines when Mandela herself threatened to sue the production and its cast unless she was allowed to see the script.

Directed by South African Darrell Roodt and based on Anna Marie du Preez Bezdrob's biography "Winnie Mandela: A Life", there were fears the film would boost the negative image of the woman who once notoriously declared that "with our boxes of matches and our necklaces, we shall liberate this country" - a reference to the placing of fuel-filled tyres around the necks of suspected apartheid informers.

Under apartheid, she was convicted of kidnapping a 14-year-old boy who was later found beaten to death in Soweto, and she later stood trial for multiple counts of fraud.

Instead, Mr Roodt opted to make the "ultimate women's movie" portraying Mrs Madikizela-Mandela as just as deserving of the praise heaped on her former husband, played by US actor Terrence Howard, for liberating South Africa from repressive white rule, the UK Telegraph reported.

"What he did was extraordinary, of course, but what she did was equally as extraordinary, if not more," Mr Roodt said when details of the film were first revealed at this year's Cannes Film Festival in May.

"Yes, he spent a long period in jail but she kept the struggle alive and was in the firing line. If it wasn't for Winnie Mandela, South Africa might have ended in ruin and chaos. It is, for me, the great untold story about South Africa."

South African actors including John Kani have protested about the film's lack of local stars, and those commenting on the leaked clip have criticized the accents of the Americans.

"Accents are terrible and the makeup is questionable. When we see the real Winnie speak, with all that passion and even at 70 she is mesmerizing, and she is both dangerous and beautiful – you can't help but love her," wrote one, named Luso, on a local paper website.

"Sorry, Jennifer, you're not Winnie, and we can see that from just a trailer."

The trailer was reportedly stolen by computer hackers from the website of the film's sales representative, Filmbridge.

"The material was not formally edited, graded or sound-mixed. At this stage, the producers have elected to make no further comment and are investigating the matter," an insider told South Africa's Times newspaper.