The BBC's director general, George Entwistle, has resigned in the wake of the Newsnight child abuse broadcast.
In a statement given outside New Broadcasting House, Mr Entwistle said: "I have decided that the honourable thing to do is to step down."
Earlier, Mr Entwistle said the Newsnight report, which wrongly implicated ex-senior Tory Lord McAlpine should never have been broadcast.
The broadcast covered cases of child abuse at north Wales care homes.
Mr Entwistle took up the post of director general on 17 September.
In his statement, he said: "In the light of the fact that the director general is also the editor in chief and ultimately responsible for all content, and in the light of the unacceptable journalistic standards of the Newsnight film broadcast on Friday 2 November, I have decided that the honourable thing to do is to step down from the post of director general."
He said that when he was appointed to the role, he was confident BBC trustees had chosen the best candidate for the post and the "right person to tackle the challenges and opportunities ahead".
"However the wholly exceptional events of the past few weeks have led me to conclude that the BBC should appoint a new leader," he said.
"To have been the director general of the BBC even for a short period, and in the most challenging of circumstances, has been a great honour.
"While there is understandable public concern over a number of issues well covered in the media - which I'm confident will be addressed by the review process - we must not lose sight of the fact that the BBC is full of people of the greatest talent and the highest integrity.
"That's what will continue to make it the finest broadcaster in the world."
During his 54 days in charge, Mr Entwistle has also had to deal with controversy over the BBC shelving a Newsnight investigation into former BBC presenter and DJ Jimmy Savile, who police say could have abused as many as 300 people over a 40-year-period.
As a result, former Sky News head Nick Pollard is examining whether there were BBC management failings following the Newsnight programme not being broadcast, Also, an inquiry has begun into the culture and practices at the BBC in the era of alleged sexual abuse by Savile. Another review is to examine sexual harassment policies at the BBC.
BBC Trust chairman Lord Patten, who made a statement following Mr Entwistle's resignation, said: "This is undoubtedly one of the saddest evenings of my public life."
He said: "At the heart of the BBC is its role as a trusted global news organisation.
"As the editor in chief of that news organisation George has very honourably offered us his resignation because of the unacceptable mistakes - the unacceptable shoddy journalism - which has caused us so much controversy.
"He has behaved as editor with huge honour and courage and would that the rest of the world always behaved the same."
Culture Secretary Maria Miller said: "It is a regrettable, but right decision. It is vital that credibility and public trust in this important national institution is restored.
"It is now crucial that the BBC puts the systems in place to ensure it can make first class news and current affairs programmes."
Tim Davie, currently chief executive of BBC Worldwide, will take over as acting director general from Sunday.
BBC home editor Mark Easton said the BBC was at a "real crossroads, because its whole future depends on convincing the public in the United Kingdom that this is an organisation in which they have confidence, and in which they have trust, and that they believe in the integrity of our news coverage".
The organisation is, in some ways, in a very dangerous position, he added.
Newsnight reported on 2 November an abuse victim's claims against a leading 1980s Tory politician.
Lord McAlpine, although not named on Newsnight, was wrongly identified on the internet as the alleged abuser at care homes in north Wales in the 1980s.
The former senior Tory has said the claims are "wholly false and seriously defamatory".
One abuse victim, Steve Messham, has apologised to Lord McAlpine, Tory treasurer during Margaret Thatcher's leadership, after saying he did not assault him.
Mr Messham said in the 1990s he was shown a photograph by police of his alleged abuser but was incorrectly told it was Lord McAlpine.
The BBC has ordered an "immediate pause" in Newsnight investigations to assess editorial robustness and a suspension of all co-productions with the Bureau of Investigative Journalism, which worked on the Newsnight broadcast.
Before his departure, Mr Entwistle had commissioned a report from BBC Scotland director Ken MacQuarrie into what happened with the Newsnight investigation.
And the BBC also ordered a senior news executive to "supervise" Friday night's edition of Newsnight, during which an apology was broadcast.--Courtesy of BBC News