The British public broadcaster has suspended all investigations by its flagship current affairs programme, Newsnight, after a politician was wrongly named on the internet as a paedophile.
The BBC apologised "unreservedly" late on Friday for having aired a Newsnight investigation alleging that a senior Conservative Party figure repeatedly abused a teenage resident of a children's home in the 1970s.
Although the BBC did not identify the politician in last week's report, Alistair McAlpine, former Conservative treasurer, was widely named on social networking sites as the alleged perpetrator.
He publicly denied the claims on Friday, and hours later his accuser Steve Messham, a former resident of the Bryn Estyn children's home in Wales, said McAlpine was not his abuser and had been a victim of mistaken identity.
"Newsnight will be back on Monday. Probably"
- Newsnight anchor, Eddie Mair
"Mr Messham has tonight made a statement that makes clear he wrongly identified his abuser and has apologised," the BBC said in a statement.
"We also apologise unreservedly for having broadcast this report."
It added that BBC Director-General George Entwistle had ordered Newsnight to air a full apology on Friday and to enact "an immediate pause in all Newsnight investigations to assess editorial robustness and supervision."
Closing Friday's edition of Newsnight, anchor Eddie Mair summed up the grim mood with the sign-off: "Newsnight will be back on Monday. Probably."
The BBC has faced a barrage of criticism in recent weeks over allegations that the late presenter Jimmy Savile, one of its biggest stars, sexually abused as many as 300 children over a 40-year period.
Newsnight itself has come under fire after it emerged that its editor shelved an investigation by the programme into the Savile allegations in December.
The BBC has launched three inquiries into the scandal, one into how Savile was able to get away with the abuse, a second into why Newsnight dropped its investigation, and a third into wider allegations of sexual harassment.
The broadcaster said on Friday that it was suspending all co-productions with the Bureau of Investigative Journalism, with whom it worked on the Bryn Estyn investigation, and ordered an urgent report into what happened.
McAlpine's lawyers say they are pursuing legal action against "all media who have defamed Lord McAlpine's reputation and published defamatory statements."
The politician, who blasted the claims as "wholly false and seriously defamatory", said he was forced to publicly deny them after he was named directly on the Internet and "by innuendo" in the written and broadcast media.
He said he had never been to any children's home, let alone the Bryn Estyn facility in Wrexham, north Wales.
"I have never stayed in a hotel in or near Wrexham, I did not own a Rolls Royce, have never had a 'Gold card' or 'Harrods card' and never wear aftershave, all of which have been alleged," McAlpine said in a statement.
"I did not sexually abuse Mr Messham or any other residents of the children's home in Wrexham."
Messham has offered his "sincere and humble" apologies to McAlpine, who served under former premier Margaret Thatcher, and suggested that it was the police who wrongly identified the politician as his alleged abuser.
In a statement published by the BBC, Messham said: "I want to offer my sincere and humble apologies to him and his family."
"The latest error could not be much more serious for the BBC's journalistic reputation," wrote one commentator for The Guardian newspaper.