Unlawful Killing, directed by the British actor - father of pop star Lily Allen, documents the 2008 inquest into the beloved royal's death in a 1997 Paris car accident, which also killed her companion Dodi Fayed and their chauffeur Henri Paul.
The documentary caused outrage in Britain after Allen chose to include a graphic black-and-white photograph of Diana moments after the crash, as rescue workers fought to save her - an image which has never been published in the U.K.
Allen defended his decision to use the grizzly snap, insisting the image was not used to shock or to boost the film's appeal.
But the controversy leading up to the movie's premiere proved irrelevant - reviewers invited to attend the big screening were left unimpressed by the piece and either left early or stayed behind to challenge Allen in a press conference about his film's criticisms and attacks on Britain's royal family.
According to the U.K.'s Independent newspaper, reporters grew angry about the documentary after Allen labelled the royals "gangsters in tiaras" and claimed that journalists never questioned the monarchy as they "wanted knighthoods".
The filmmaker also reportedly became embroiled in an argument with Martyn Gregor, writer of a biography on the Princess of Wales' death, who heckled Allen throughout the premiere.
Daily Mail critic Baz Bamigboye has also dismissed the film as "unlawfully dull" and slammed Allen for approaching Dodi's businessman father Mohammed Al-Fayed for financial backing, insisting the documentary is nothing more than "the ramblings of a sour old man who misses his son".