BBC staff were tonight facing the prospect of disciplinary action amid growing anger over the broadcast of child abuse allegations which appears to have led to a former senior Conservative being falsely implicated.
The corporation's director general George Entwistle today admitted the Newsnight report which appears to have led to Lord McAlpine of West Green being falsely identified as a paedophile should never have been shown.
With ministers and MPs demanding an explanation as to why the latest crisis to hit the BBC had erupted so soon after the Jimmy Savile child abuse scandal, Mr Entwistle promised swift action against anyone found to be at fault.
An urgent report he commissioned from BBC Scotland director Ken MacQuarrie into the Newsnight report is due on his desk tomorrow.
"Further action will follow from that - disciplinary if necessary," he said.
However, Mr Entwistle was forced to defend his own position after he disclosed that he had been unaware of the Newsnight report until after it had been shown. But he said he would not be resigning over the issue.
MPs expressed concern that new procedures he put in place after the Savile affair - when the BBC was panned for not running a Newsnight report exposing the late DJ as a child abuser - had still not resolved the problems in the corporation's management structure.
The chairman of the Commons Culture, Media and Sport Committee, John Whittingdale, said someone would have to be held accountable for the latest fiasco.
"I certainly think somebody needs to take responsibility for this," he said.
The Newsnight report dramatically unravelled last night when child abuse victim Steve Messham said Lord McAlpine was not the man who had abused him when he was a teenager at a North Wales children's home in the 1970s and 1980s.
Although the programme, shown on November 2, had not named the peer - referring only to a senior Conservative from the Thatcher era - it quickly resulted in him being identified on internet blogs and social media sites.
Last night, with lawyers for Lord McAlpine indicating they were preparing to sue for defamation, the programme broadcast an on-air apology.
In back-to-back interviews this morning on the BBC Radio 4 Today programme and BBC1's Breakfast, Mr Entwistle acknowledged that the BBC was facing a "bad crisis of trust".
He said it was now vital to establish how a report which was so "fundamentally wrong" ever reached the screen, despite having been referred to lawyers and the BBC board of management.
Among the issues he said needed to be resolved was whether Mr Messham was actually shown a photograph of Lord McAlpine, and whether the peer had been given a chance to respond to the allegations.
Despite the storm over the dropping of Newsnight's Savile investigation, Mr Entwistle said he had not been alerted that the programme was planning to make fresh allegations over another public figure.
He said he had been unaware of a tweet 12 hours before the programme was broadcast, 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 said.
"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."
Even yesterday he said he had been unaware of a report in The Guardian naming Lord McAlpine and suggesting he was a victim of mistaken identity, because, he said, he had been preparing for a speech.
At one point during a bruising encounter with Today presenter John Humphrys, he was challenged as to why he failed to take a more active interest in what was happening - even though he was the BBC's editor-in-chief.
"So there is no natural curiosity, you wait for somebody to come along to you and say 'Excuse me director general, but this is happening and you may be interested'?" Mr Humphrys demanded.
"You don't look for yourself, you don't do what everybody else in the country does, read newspapers, listen to everything that's going on and say 'What's happening here?"'
Mr Entwistle said he only became involved if a programme was referred to him for his attention.
He insisted he had no intention of resigning, although he accepted his future now lay in the hands of his employers the BBC Trust.
"I am doing the right things to try and put this stuff straight. I am accountable to the Trust in that endeavour. If they do not feel I am doing the right things, then obviously I will be bound by their judgment," he said.
In a statement, the Trust said it had been a "deeply troubling episode" and that it expected Mr Entwistle to get to the bottom of what happened and take the "appropriate action" as quickly as possible.
Some MPs were openly questioning whether Mr Entwistle could carry on as director general.
Tory Rob Wilson, a prominent critic of the BBC over the Savile affair, said: "I think there are questions for George Entwistle about whether he is the right person to lead the BBC out of the difficulty and the crisis it finds itself in."
Mr Whittingdale said somebody would have to carry the can for what had happened - although he stopped short of calling for Mr Entwistle to resign.
"At the end of the day, the director general of the BBC is editor-in-chief. I would have expected a programme making as serious allegations as these to have gone to him for clearance," he said.
"This has done immense damage to the reputation of the BBC. I certainly think somebody needs to take responsibility for this."
Culture Secretary Maria Miller said events of the past days underlined the "vital importance of restoring credibility" in the corporation.
Labour deputy leader and shadow culture secretary Harriet Harman said it was clear something had 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," she said.
In a statement tonight the trustees of the Bureau of Investigative Journalism said they were "appalled" at what appeared to have been 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," the statement said.