The BBC's director-general has said he has given no thought to axing Newsnight over a report that wrongly implicated a former senior Tory in a child abuse scandal.
Grilled by BBC presenter John Humphrys, George Entwistle admitted he did not know about the Newsnight investigation until the day after it was broadcast and had not seen newspaper reports casting doubts on the probe.
The BBC boss said he had also been unaware of a tweet 12 hours before the programme aired, from the Bureau of Investigative Journalism - which worked on the report - suggesting it was going to identify a senior political figure.
"I didn't see that tweet. This tweet was not brought to my attention so I found out about this film after it had gone out," he told the BBC's Today programme.
"In the light of what has happened here I wish this was referred to me, but it wasn't. I found out about the film the following day."
James Lee, the chairman of the Trustees of the Bureau of Investigative Journalism, has released a statement saying: "The Trustees are appalled at what appears to be a breach of its standards."
"To the extent that the principles of The Bureau have been ignored by an involvement in this story, remedial action will be taken against those responsible. The Trustees must ensure that due process is applied and are establishing the key facts."
Mr Entwistle said the 32-year-old Newsnight programme had "a fantastic investigative record" and it would be "disproportionate at this stage to talk about closing Newsnight down".
However, he said the report on child abuse allegations was "unacceptable" and "should never have gone out", warning that staff involved in the programme could now face disciplinary action.
"We should not have put out a film that was so fundamentally wrong. What happened here was completely unacceptable," he said.
"I have taken clear and decisive action to start to find out what happened and put things right."
Mr Entwistle, who had moved quickly to try to limit the damage by appointing BBC Scotland director Ken MacQuarrie to produce a report into why basic errors were made, said he expected it to be on his desk by Sunday.
Pressed about his own position, he insisted he had no intention of resigning - although he accepted his future lay in the hands of the BBC Trust, which described the report as a "deeply troubling episode".
The Trust also offered its own apology in a statement, adding it had "impressed upon the director-general the need to get to the bottom of this as a matter of the utmost urgency and will expect appropriate action to be taken as quickly as possible".
Responding to the BBC investigation into the Newsnight report, Harriet Harman, Labour's Shadow Secretary of State for Culture, Media and Sport said: "Even before we learn the results of the urgent report by BBC Scotland director Ken MacQuarrie, commissioned by the director-general, it is absolutely clear that something has gone badly wrong at Newsnight."
"The director-general only took over the leadership of the BBC eight weeks ago, but he needs to show decisively that he is addressing the systemic problems which are in evidence here."
Kevin Marsh, a former editor of the Today programme and BBC College of Journalism, said it was "extraordinary" Mr Entwistle was unaware of the report following the storm over the dropping of Newsnight's investigation into Jimmy Savile.
"It was quite grim for him before his interview, but I think it's even grimmer after," said Mr Marsh.
"I think many people will find it extraordinary that he hadn't put out an alert to his senior managers saying, 'Look, if Newsnight and child abuse come together again, I need to know,' and 'I want to know all the details'."
MP John Whittingdale, chairman of the Commons Culture, Media and Sport select committee, described the debacle as "the most appalling failure of management at every level of the BBC".
Despite a number of requests, the BBC told Sky News it would not be making Mr Entwistle, or any other executive, available for interview on Saturday.
Newsnight made an unreserved apology on air on Friday night for the broadcast on November 2 after Steve Messham admitted the man who abused him as a teenager at a care home in North Wales was not former Conservative Party treasurer Lord McAlpine.
The 70-year-old peer found himself at the centre of a storm of internet speculation after Mr Messham told the BBC2 programme he had been abused by a senior Conservative from the Thatcher era.
There will also be a "pause" in all ongoing Newsnight investigations, while the BBC is suspending all co-productions with the Bureau of Investigative Journalism.
Nevertheless, the disclosures come as another blow for the corporation, which is still reeling from the Savile scandal - including a decision to drop a Newsnight investigation exposing the late DJ as a child abuser.
Tory MP Rob Wilson said Mr MacQuarrie needed to examine whether the programme on the Bryn Estyn children's home scandal had been an "over-compensation" for what happened over Savile or a "diversionary tactic" to draw attention away from the BBC.
The programme had included an interview with Mr Messham, who described how he used to be taken from the children's home to a hotel near Wrexham to be abused by men, including one described as a former senior Conservative.
On Friday, Lord McAlpine broke cover to issue a vehement public denial of the "wholly false and seriously defamatory" claims against him. His solicitors have indicated they are preparing to sue for defamation.