The BBC has issued an unreserved apology for a Newsnight report which led to Lord McAlpine being wrongly implicated in the alleged sexual abuse of children at north Wales care homes.
It comes after a victim, Steve Messham, apologised to the former Tory treasurer after saying he did not assault him.
Newsnight had reported Mr Messham's claims against a leading 1980s Tory politician but did not name him.
The BBC also announced an "immediate pause" in all Newsnight investigations.
Earlier, Lord McAlpine said the claims were "wholly false and seriously defamatory".
His solicitor, Andrew Reid, said the peer will take legal action against those who later named and linked him to the false allegations.
The BBC's statement, issued on Friday evening and also broadcast at the beginning of Friday's Newsnight programme, said: "On 2 November, Newsnight broadcast a report that looked into criticism of the North Wales Abuse Tribunal.
"The report included an interview with Steve Messham, an abuse victim who said that a senior political figure of the time had abused him.
"We broadcast Mr Messham's claim but did not identify the individual concerned. Mr Messham has tonight made a statement that makes clear he wrongly identified his abuser and has apologised.
"We also apologise unreservedly for having broadcast this report."
The BBC said that, on Friday night, director general George Entwistle had ordered the following actions:
- Sending in a senior news executive to "supervise" Friday night's edition of Newsnight
- An apology to be broadcast on Friday's programme
- An "immediate pause in all Newsnight investigations to assess editorial robustness and supervision"
- Commissioning BBC Scotland director Ken MacQuarrie to write an "urgent report for the DG covering what happened on this Newsnight investigation"
- An "immediate suspension" of all co-productions with the Bureau of Investigative Journalism which Newsnight worked with on the programme
Rob Wilson, Conservative MP for Reading East, told the BBC the Newsnight report "looked like a pretty shoddy piece of journalism to me, and yet the editorial process wasn't robust enough to pick that up and deal with it".
"Despite the fact that Newsnight has had many fabulous journalists, and done many great stories, this is really its darkest hour."
Mr Messham, a former resident of the Bryn Estyn home, last week told Newsnight he had been abused by a senior politician of the Thatcher era.
He called for a new investigation claiming a report by Sir Ronald Waterhouse in 2000 had not uncovered the full scale of the abuse.
Mr Messham said police had shown him a picture of his abuser but incorrectly told him the man was Lord McAlpine.
He has offered "sincere and humble apologies" to the peer and his family.
In a statement issued early on Friday evening Mr Messham said: "After seeing a picture in the past hour of the individual concerned, this [is] not the person I identified by photograph presented to me by the police in the early 1990s, who told me the man in the photograph was Lord McAlpine."
He later told BBC News he was "actually mortified" when he realised.
He said recent coverage of the scandal had "brought everything back to the fore but I just want justice for everybody and hope we get justice for everybody".
"But I certainly don't want the wrong people accused, that is also wrong.
"That's why I'm speaking out now and I hope people believe what I say. I am sincerely sorry."
North Wales Police said it would be inappropriate to comment on Mr Messham's allegations about the mix-up over identifying the picture "because an investigation is ongoing".
Earlier Lord McAlpine's solicitor, Andrew Reid, said the peer had "no choice" but to take legal action over claims linking him to abuse at Bryn Estyn children's home in Wrexham.
The solicitor also criticised the way Newsnight aired Mr Messham's claims last week.
Lord McAlpine, who was Conservative Party treasurer from the late 1970s until 1990, said: "I have never been to the children's home in Wrexham, nor have I ever visited any children's home, reform school or any other institution of a similar nature."
Lord McAlpine, whose grandfather founded the McAlpine construction firm, is a life peer who took the title of Baron McAlpine of West Green in Hampshire.
He became Conservative Party Treasurer in the late 1970s and held the post until 1990. He now lives in Italy.
Lord McAlpine said: "I do not suggest that Mr Messham is malicious in making the allegations of sexual abuse about me.
"If he does think I am the man who abused him all those years ago I can only suggest that he is mistaken and that he has identified the wrong person."
The BBC has commissioned several inquiries already:
- Ex-Sky News boss Nick Pollard is looking into why Newsnight dropped an investigation into sexual abuse claims against former BBC presenter Jimmy Savile last year
- Former Appeal Court judge Dame Janet Smith will investigate whether culture and practice at the BBC enabled Savile to carry out the sexual abuse of children
- A BBC investigation led by Dinah Rose QC is looking into handling of past sexual harassment claims
BBC media correspondent Torin Douglas said the McAlpine apology was "very damaging" to Newsnight itself and the BBC as a whole, coming as it does during the Savile scandal.
He said one issue to consider was whether Newsnight felt it had to run the story because it did not run the Savile programme.
On Tuesday the Home Secretary Theresa May announced a new police inquiry into the allegations of child abuse in north Wales in the 1970s and 1980s.
Mrs May said the head of the National Crime Agency would lead the inquiry.
Separately Mrs Justice Macur will investigate the terms of the Waterhouse abuse inquiry, which began in 1996.
The Home Office said there would be no change to the inquiries announced by Mrs May, following the statements by Mr Messham and Lord McAlpine.