The actor and filmmaker, who has appeared in Robin Hood, Shallow Grave and Trainspotting, is to debut Unlawful Killing on May 13 after working on the project for three years.
The feature will mix interviews with recreations of the official inquest into the Paris car crash that killed the Princess, her boyfriend Dodi Fayed and their driver Henri Paul. The film includes contributions from the late Tony Curtis, Howard Stern, Kitty Kelley, Mohamed Al Fayed, Piers Morgan and Michael Mansfield QC, while Dave Stewart provides an original score.
The documentary alleges that "dark forces" within the British establishment rushed to cover up Diana's death in 1997. Its premiere this Friday will coincide with what would have been the Princess of Wales's 50th Birthday.
Allen said of Unlawful Killing's Cannes debut: "Screening this film in Cannes for the world's media will be both exhilarating and terrifying for me. As far back as 2004, I had been intrigued by Mohammed Al Fayed's unrelenting determination to seek answers to the questions surrounding the death of his son, Dodi and Princess Diana.
"By going 'undercover' at the inquest, I hoped to reconcile some of my own suspicions too - but what I experienced was horrifying. This film is, in short, the inquest of the inquest."
The 64th Cannes Film Festival runs from May 11 to May 20.
Watch the trailer for Unlawful Killing below: