The Newsnight report that led to Lord McAlpine being wrongly named as a paedophile aired largely because BBC staff failed to follow the corporation's own editorial guidelines, the BBC Trust has said.
The November 2 broadcast was a "grave breach which had been costly to all concerned" and resulted in the public being misled, the Trust's Editorial Standards Committee (ESC) found.
The committee announced it has ordered another report by the BBC next year to determine what the corporation must do to "(learn) from these events".
Acting BBC Director-General Tim Davie already confirmed Adrian van Klaveren, the BBC Radio 5Live controller, will be moved to a new role outside of news following the report.
Mr Van Klaveren was the acting head of news at the time the Newsnight report ran.
The now-infamous programme, which featured an interview with Steve Messham who said a senior political figure of the time abused him, led to the widespread naming of Lord McAlpine.
The Tory peer, who had not been contacted by the programme, strenuously denied the allegations, launching legal action.
The following week, Newsnight broadcast an apology, as well as apologies from Mr Messham, and Lord McAlpine later received £185,000 damages from the corporation.
An internal BBC report, led by the corporation's Scotland director, Ken MacQuarrie, found that Newsnight failed to complete "basic journalistic checks" and there was confusion about who had the ultimate responsibility for "final editorial sign-off".
He said the programme's editorial management structure had been "seriously weakened" after the editor stepped aside over the Savile scandal and the deputy editor departed.
Today's ESC report condemned a failure of editorial control within the BBC, which resulted in a "most serious breach of the accuracy guidelines":
This was a high-risk report which required rigorous supervision and did not receive it.
The allegations were not based on sound evidence.
They were not thoroughly tested and, whilst there was no suggestion that the programme-makers had sought to mislead the public, this had been the effect.
The combination of the Newsnight report and the online speculation had led to Lord McAlpine being incorrectly identified as Mr Messham's abuser.
The Trustees noted that the failures by members of the Newsnight team resulted largely from a failure to adhere to the editorial guidelines.
The committee said issues of privacy and fairness obligations were being considered by Ofcom, so it would turn to those after the watchdog had completed its investigation.
An Ofcom spokesman confirmed its inquiry was "ongoing" with the outcome to be published "in due course":