Its makers claim the film throws light on circumstances surrounding the death of Diana, Princess of Wales in 1997.
But at a stormy media conference after its premiere, director Keith Allen was repeatedly asked to explain what new evidence he believes the film contains.
Allen disagreed with suggestions he should have made it clear on screen that the film was funded by the businessman Mohamed Al Fayed, saying it would have interrupted the film’s flow.
Conor Nolan, Mr Al Fayed’s spokesman, said the former owner of Harrods had invested £2.5m in the documentary.
Mr Al Fayed’s son, Dodi, was also killed in the crash as well as driver Henri Paul. The sole survivor, Trevor Rees-Jones, does not feature in the film.
In its structure, Unlawful Killing is like many documentaries seen on TV.
Narrated by Allen, it is a mixture of archive footage, new interviews, graphics and reconstructions of scenes from the 2007 inquest in London.
Only music by Dave Stewart gives a slightly more cinematic feel.
