The broadcaster aired the trailer for the film during an episode of the US drama Homeland in May, then encouraged viewers to send in their reactions via Twitter. These live reactions were subsequently broadcast in the following commercial break.
According to The Guardian, Ofcom investigated the promotion after a member of the public complained that the coverage "confused the viewer about whether they were watching an impartial continuity announcement or a paid advertisement".
The media regulator upheld the complaint, and told Channel 4 that the use of one of the broadcaster's most recognised announcers, along with the language used in the advert and the use of the Channel 4 logo, suggested that the channel owned and endorsed the material.
In a statement, it claimed: "In Ofcom's view, such viewer interaction is more commonly associated with television programmes than with advertising. We consider that the presentation style of the Prometheus material risked confusing viewers in respect of its status."
Channel 4 claimed that there was a clear contrast between the identification of the channel and the start of a commercial break, but Ofcom dismissed the response, saying: "Simply because material appears in a break between programmes is not sufficient to identify it as advertising."
The broadcaster has now implemented a new internal policy to prevent a repeat of the incident, which will include the broadcaster's legal and compliance team being required to sign off any potentially troublesome ads.
Watch the Prometheus trailer aired during Homeland below: