The BBC engaged in a "major breach" of broadcasting rules by not revealing that the presenter of a film about the Gaza conflict was the offspring of a Hamas official, as ruled by UK media regulator Ofcom.
An inquiry by Ofcom into Gaza: How to Survive a Warzone has determined that the film was "materially misleading".
The head of the BBC had earlier expressed regret, saying there had been "a serious error in relation to factual correctness".
Ofcom has instructed the broadcaster to air a prime-time declaration about its findings.
"Since this amounts to a serious breach of our rules, we are directing the corporation to transmit a statement of our determinations against it on BBC2 at 9pm, with a schedule to be determined," it said.
'Risk to Erode Confidence'
The watchdog's report stated: "The inquiry we conducted concluded that the film's failure to disclose that the narrator's father occupied a position in the Hamas-run authority was substantially inaccurate.
"Consequently that the viewers did not have essential details which may have been highly relevant to their evaluation of the voice and the details he presented.
"Trust is at the core of the bond between a broadcaster and its audience, especially for a public service broadcaster such as the BBC.
"This oversight had the risk to erode the very considerable degrees of confidence that audiences would have put in a documentary produced by the BBC about the conflict in Gaza."
The film was withdrawn from the BBC's streaming service in February after the narrator's familial connections were revealed, and in the month of July an broadcaster's internal assessment determined it breached the corporation's editorial guidelines on truthfulness.
A BBC spokesperson commented: "The Ofcom ruling is in accordance with the results of the internal assessment, that there was a serious error in the documentary in relation to the BBC's Editorial Guidelines on truthfulness, which aligns with clause 2.2 of the authority's broadcasting regulations.
"We expressed regret for this and we comply with the regulator's ruling in its entirety. We will adhere to the sanction as soon as the date and language are finalised."
