The BBC Trust has a listed a catalogue of failures which led to Newsnight reporting false claims that a senior Conservative politician was a paedophile.
The broadcast on 2 November 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.
Its report, published today, coincided with the Pollard Review into why a Newsnight investigation into broadcaster and DJ Jimmy Savile was shelved.
And in its findings, the committee ordered another report by the BBC next year into what steps will be taken to make sure the corporation "learns from these events".
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The 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.
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A report by the BBC, 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 found that the programme's editorial management structure had been "seriously weakened" as a result of the editor having to step aside over the Savile scandal, and the departure of the deputy editor.
Today, the ESC said there was a failure of editorial control within the BBC, adding: "This was a high-risk report which required rigorous supervision and did not receive it."
The committee's report said a "most serious breach of the accuracy guidelines had occurred".
"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," Trustees said.
"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.
It has requested a report from the BBC next year "on the steps that will be taken to ensure the BBC learns from these events after Ofcom has published its finding".
"The Executive will also be asked to identify where best practice and experience in investigation and editorial control exists in the BBC and share those lessons across the BBC. This report will be published."
It added: "This has been a grave breach which had been costly to all concerned.
"The Trustees wish to put on record an apology to Lord McAlpine and to the public for this serious failure of BBC journalism."
The botched investigation led to the resignation of director-general George Entwistle after just 54 days in the job, and three BBC employees have been disciplined.
It also prompted the corporation to suspend all new investigations by Newsnight, as well as put on hold work with the Bureau of Investigative Journalism (BIJ), which helped in the programme.
Twelve hours before the Newsnight report was broadcast, then- managing editor of the BIJ Iain Overton – who has since resigned – tweeted: "If all goes well we've got a Newsnight out tonight about a very senior political figure who is a paedophile", leading to further speculation throughout the day of broadcast.
Today's report from the Trust's Editorial and Standards Committee said the failure by Newsnight was "extremely concerning".
It found that freelance reporter Angus Stickler, who took the story to the programme, was considered a "safe pair of hands" and may have been subjected to "lighter-touch editorial checks" than if he had been less well-known to the BBC.
It said the report was wrongly treated as if it had two sources – Mr Messham, and an interview with a second victim broadcast on BBC Five Live in 2000, who could not be contacted.
"The Trustees agreed that members of the Newsnight team were wrong to regard this as a report with two sources, given that the second witness could not be found.
"The Trustees found it particularly concerning that, at no point in advance of the broadcast of the Newsnight report, was Mr Messham shown a photograph of Lord McAlpine and asked to confirm that he was the individual at the centre of the allegations."
It found that, although Lord McAlpine had denied the allegations when contacted by Channel 4 News journalist Michael Crick ahead of the broadcast of the Newsnight programme, the BBC still made no attempt to contact the peer, which it should have done.
Trustees also said that concerns had been raised previously about the credibility of some of Mr Messham's evidence – which was known to members of BBC Wales, but no effort was made to contact them.
"The Trustees considered that parts of BBC News not contacting colleagues in the nations and regions when investigating stories on their patch was very poor practice."
They also noted that the failure came in the wake of the decision not to run the Savile Newsnight programme in December 2011 and said that situation led to a "lack of clarity concerning who had overall editorial responsibility for the content of the Newsnight report".
They said: "Concern about possible reputational damage in that event 'played too large a part' in the decision to proceed with the broadcast."
The committee found there was a lack of clarity in who had final editorial sign-off.
They added: "The Trustees were clear that there was a failure of editorial control within the BBC; this was a high-risk report which required rigorous supervision and did not receive it."
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