A report into a BBC programme which mistakenly implicated a senior politician in a child abuse scandal has found a number of "unacceptable" failings.
Ken MacQuarrie, director of BBC Scotland, was asked to investigate the November 2 edition of Newsnight, which led to Lord McAlpine, the former Conservative treasurer, being wrongly accused of abuse at a care home in North Wales in the 1970s and 1980s.
His report found that "some of the basic journalistic checks were not completed", and that there was "ambiguity around who was taking the ultimate editorial responsibility".
The Corporation said it would take "clear and decisive action ... to restore public trust in the BBC's journalism", adding disciplinary proceedings "will begin immediately".
The BBC Trust described Mr MacQuarrie's findings as "very concerning", and said "serious failures in the normal checks and balances that the Trust expects from BBC journalism" had been identified.
A spokesman for the BBC said: "The next stage of this process is now underway.
"It will, amongst other things, seek to clarify decision-making roles and responsibilities in relation to the Newsnight report.
"We expect that these investigations will be concluded as quickly as possible."
Newsnight did not name Lord McAlpine in its report but speculation on the internet forced him to issue a statement on Friday, denying his involvement.
George Entwistle resigned as the BBC's director-general one day later and will receive a £450,000 pay-off - a year's full salary in lieu of notice after just 54 days in the post.
Under the terms of his contract Mr Entwistle was entitled to only six months' pay.
However, the BBC Trust said the additional payment had been agreed as a reflection of his continuing involvement with the various BBC inquiries now under way.
Prime Minister David Cameron said the amount was "hard to justify", while Culture Secretary Maria Miller told the House of Commons that the National Audit Office has the power to undertake a value-for-money review into the pay-off.
Meanwhile, Tim Davie, the BBC's acting director-general, told Sky News he has begun the job of rebuilding trust in the public broadcaster.
He said: "The key here is to have a very clear line of journalistic control to me.
"This is about establishing clear lines of responsibility in our journalism and I think I'm in a good position to do that.
"We've had a major problem over the last few weeks. I need to grip that, but the people I'm putting in charge I absolutely trust and they are very strong journalists.
"We need to rebuild trust and that is what I'm going to do."
In other developments, the BBC's director of news, Helen Boaden, and her deputy, Stephen Mitchell, both stepped aside, although neither were involved in the Newsnight investigation.
Iain Overton has resigned as editor of the Bureau of Investigative Journalism, which worked with the BBC on the report.
His tweet that a programme "about a very senior political figure who is a paedophile" was about to be broadcast was retweeted more than 1,500 times.