London: The BBC's Director-General George Entwistle resigned after the broadcaster's flagship news programme wrongly implicated a British politician in child sex abuse. (Agencies)
"I have decided that the honourable thing to do is to step down," Entwistle said in a televised statement outside the BBC's London headquarters late on Saturday.
Entwistle, who only took over as director-general in September, said he decided to resign "in the light of the fact that the director-general is also the editor-in-chief and ultimately responsible for all content".
"The wholly exceptional events of the past few weeks have led me to conclude that the BBC should appoint a new leader," he added.
"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."
Tim Davie, who is currently the BBC's director of audio and music, will take over as acting director general.
Earlier on Saturday, Entwistle had said it was "fundamentally wrong" of the current affairs programme Newsnight to air an interview with a man claiming he was repeatedly abused by a senior Conservative politician at a children's home in the 1970s.
The programme did not identify the politician in last week's report, but he was widely named on the Internet as former Tory party treasurer Alistair McAlpine.
McAlpine went public on Friday to strongly deny the allegations, and hours later his accuser Steve Messham retracted his claims, saying he had only just seen a picture of McAlpine and he was not the man who abused him.
Entwistle's resignation comes just weeks after a storm of allegations that the late BBC presenter Jimmy Savile, one of the corporation's biggest names, sexually abused hundreds of children over four decades.
Both Newsnight and the director-general were already under scrutiny after the show dropped an investigation into the Savile abuse claims last year.
Entwistle who edited Newsnight himself a decade ago had admitted before his resignation that the corporation faced a "crisis of trust" over the Newsnight broadcast and the Savile scandal.
Chris Patten, the chairman of the BBC's governing body, the BBC Trust, said it was "one of the saddest evenings of my public life".
Standing alongside Entwistle as he made his resignation statement, the former Hong Kong governor said: "At the heart of the BBC is its role as a trusted global news organisation.
Entwistle's resignation follows a day of heavy criticism which saw the director general mauled on the Radio 4's On Sunday programme by John Humphreys, who probed the recently appointed editor-in-chief as to why he hadn't taken more of an interest into the programme's output, particularly in the wake of the Jimmy Savile scandal.
Entwistle told Humphreys: "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."
In a brutal examination of his boss's failings, Humphrey barked: "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'?"
"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?'"
On Saturday evening, Labour deputy leader and shadow culture secretary Harriet Harman waded into the row, emphasising that 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.
Throughout the day, pundits and politicians lined up to point out the failings of the programme, its staff and the director general, including a broadside from parliamentary culture select committee chairman John Whittingdale, who slammed the BBC chief's "extraordinary lack of curiosity" in Newsnight's investigations.
During his savaging on the On Sunday programme, Entwistle apologised for Newsnight's investigation leading to the incorrect "outing" of Lord McAlpine as the senior Tory paedophile, calling the reports "unacceptable", and announcing he would look into the affair and that he had "taken clear and decisive action to start to find out what happened and put things right".\
London: The BBC's Director-General George Entwistle resigned after the broadcaster's flagship news programme wrongly implicated a British politician in child sex abuse.