The film focuses partly on the inquest into Diana's death and argues the press failed to properly reflect its findings due to pressure from the royal family.
Director Keith Allen on Friday defended his film about Princess Diana's death, amid accusations that it was a one-sided attack on what he called the British "establishment" and entirely funded by Mohamed al-Fayed.
The businessman, whose son Dodi died with Diana in a 1997 Paris car crash, has long maintained that the couple was killed on the orders of queen Elizabeth's husband, Prince Philip, believing the royal family did not want Diana marrying a Muslim.
At a sometimes heated press conference in Cannes, where Allen is launching Unlawful Killing outside the official film festival, he described the documentary as "forensic", a description some attendees questioned.
The film focuses partly on the 2007-8 inquest into Diana's death and argues the British press failed to properly reflect its findings due to indirect pressure from the royal family.
"I thought it was important that the public got to understand in a forensic manner what was happening in this inquest," Allen, best known as a British television actor, told reporters in Cannes.
"I didn't want to make a sensationalist film, I don't think it is a sensationalist film. I think it is a very forensic analysis of a British legal process and I think it reveals certain things that ... don't add up.
"I believe that they should be questioned. That's why I made the film. I hope it shows people that nothing is at it seems."
Allen was directly criticised by one journalist for failing to make clear that the film was entirely funded by Fayed to the tune of 2.5 million pounds ($4.1 million), a figure provided by a man claiming to be Fayed''s representative.
Fayed was not in Cannes to present the film.
"He put money in because nobody else would," Allen said. "If I could have got it somewhere else I would have got it somewhere else. But I didn't, I got it off him."
Investigations by French and British police have concluded the deaths of Diana and Dodi were a tragic accident caused by a speeding chauffeur, who was found to be drunk. They have both rejected Fayed''s theories.
Asked what was new in Unlawful Killing, Allen added: "I don't believe that there is too much that is new.
"There's an old saying in our country which is the best kept secrets are on the bookshelves of the British Library. They're all there if you care to go and look for them.
"This is not an attack on the monarchy. It's actually questioning the role of the establishment and in so doing it will make it clear that there are connections between the royal House of Windsor and the establishment."
The documentary includes a photograph of Diana dying after the car crash, an element which the British press focused on in recent days, although Allen played it down.
"As I said when I presented the film, there won't be a sharp intake of breath when you see the photograph of Princess Di. It's nowhere near as sensational or revealing as people made it out to be."
Allen was still not sure whether the documentary would be shown in Britain due to legal issues.
"When you want to screen a film in England, you have to have insurance, and the only way that you can get insurance is if it's lawyer-approved. I could get lawyer approval if I'd made 87 cuts which I wasn't prepared to make."
One early review was damning. "Princess Diana film Unlawful Killing is just unlawfully dull," wrote Baz Bamigboye, albeit in the Daily Mail, deemed an "establishment" newspaper.
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