Guardian columnist George Monbiot has issued an "abject" apology for smearing Lord McAlpine during frenzied speculation about the identity of a paedophile politician.
The former treasurer of the Conservative party took legal action after being named on the web as the leading Tory who was a member of an alleged paedophile ring at a north Wales care home.
Comments on Twitter and Facebook fuelled what became a witch-hunt. Monbiot joined in by tweeting: "I looked up Lord #McAlpine on t'internet. It says the strangest things."
Monbiot claimed he thought he was doing the right thing when he directed his 55,000 Twitter followers to baseless internet claims about the 70-year-old McAlpine, who is in poor health.
Wrongly naming McApline as a paedophile or linking his name via innuendo to a child sex scandal in north Wales could result in huge compensation claims against those guilty of defaming the former MP.
The BBC has already paid out £185,000 in damages, and more could follow from other parties.
Monbiot admitted: "I allowed myself to be carried away by a sense of moral outrage. As a result, far from addressing an awful injustice, I contributed to one."
Below is Monbiot's full apology to McAlpine.
"I have helped to malign an innocent man. I have done a few stupid things in my life, but nothing as stupid as this. The tweets I sent which hinted - as I assumed to be the case - that Lord McAlpine was the person the child abuse victim Steve Messham was talking about were so idiotic that, looking back on them today, I cannot believe that I wrote them. But I did and they are unforgiveable.
"I helped to stoke an atmosphere of febrile innuendo around an innocent man and I am desperately sorry for the harm I have done him. I have set out, throughout my adult life, to try to do good; instead I have now played a part in inflicting a terrible hurt upon someone who had done none of the harm of which he was wrongly accused. I apologise abjectly and unreservedly to Lord McAlpine.
"What follows is in no sense an attempt to excuse the tweets I wrote, but simply to try to explain them. I knew that Steve Messham had been treated appallingly and I believed that the terrible things done to him had been compounded by a denial of recognition and a denial of the recourse to the law which was his due. When I saw his interview on Newsnight I was very upset.
"I trusted his account unquestioningly. I was horrified by what he said and by the fact that the identity of the man he was talking about appeared to have been kept secret for so long. I felt a powerful compulsion to do what I have done throughout my career; to help the voiceless to be heard. But in this case I did so without any of the care I usually take when assessing and reporting an issue.
"I allowed myself to be carried away by a sense of moral outrage. As a result, far from addressing an awful injustice, I contributed to one. I have acted in an unprofessional, thoughtless and cruel manner and I am sorry beyond words."