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The 90-minute documentary, "Unlawful Killing," by British actor and director Keith Allen includes a black and white close-up image of Diana shortly after the 1997 car accident in a Paris underpass that killed her.
The distressing paparazzi image has never been publicly seen in the UK, with Diana's face heavily pixelated when similar pictures were shown at the inquest into her death.
Mohammed al Fayed -- whose son Dodi also died in the accident -- has reportedly financially backed the film. A spokesman said, however, that the former Harrods department store owner had not been aware the photograph would be shown.
"He is appalled by that and will be taking all necessary steps to make sure it is not in the film," the spokesman said.
Princes William and Harry have declined to comment but royal sources told the Daily Mail the pair were sickened.
"They rather hope people would treat this with the contempt it deserves," the source said.
The film exploring Diana's death will premiere at the famous French film festival this week because British lawyers insisted on 87 cuts to the film, according to Allen.
It will be shown, he said, in the US and around the world, but will not be screened in the UK.