A Harry Potter day on Radio 1 which coincided with the release date of the franchise's latest film gave it undue prominence, the BBC Trust has ruled.
It said this was because of "so many trails and mentions" of Harry Potter and the Half-Blood Prince.
References "were not used sparingly" in output aimed at children, it added.
The trust, which said individual shows complied with guidelines, partially upheld the complaint from commercial radio industry body RadioCentre.
It had complained to BBC management twice before the matter was referred to the corporation's governing body.
The Harry Potter coverage ran from 0630 to 1900 on 15 July 2009 across the shows of Chris Moyles, Sara Cox, Edith Bowman and Scott Mills.
The BBC Trust said the corporation's editorial guidelines were breached by the day's programming as a whole.
"The verbal references to the brand or the film were not used sparingly nor were there very strong journalistic reasons for repeated references," the trust said in its report.
It added: "The number of references and the manner in which the trails were delivered amounted to undue prominence".
But it rejected RadioCentre's assertions that the coverage amounted to an endorsement of the film or that a BBC website set up to promote the day gave undue prominence to it.
