The raunchy scene in the 1973 film has become the stuff of Hollywood legend, despite denials from the stars and producer Michael Deeley, who spoke out in 2008 in a bid to quash the sensational story.
But the saga has come to the fore again thanks to former Variety editor Peter Bart's new book, Infamous Players: A Tale of Movies, the Mob, (and Sex).
Bart claims he saw Sutherland and Christie filming the thriller's bedroom scene, explaining, "It was clear to me they were no longer simply acting: they were f**king on camera."
But Sutherland has spoken out to dismiss Bart's version of events, insisting the writer was not even on set at the time the scene was shot.
In a statement obtained by Reuters, he says, "Not True. None of it. Not the sex. Not him witnessing it. From beginning to end there were four people in that room. Nic Roeg (director), Tony Richmond (cinematographer), Julie Christie and me. No one else."
Sutherland and Christie were rumoured to have enjoyed an onset affair and the actress' then-boyfriend Warren Beatty was reportedly so upset about the saucy scene, he demanded movie bosses cut the footage.