The high court rules that a tweet sent by Sally Bercow, the wife of the Commons speaker, about Conservative peer Lord McAlpine was libellous.
Ms Bercow had denied that what she had written libelled the peer and said after the judgment that she was “surprised and disappointed”.
She added: “However, I will accept the ruling as the end of the matter. I remain sorry for the distress I have caused Lord McAlpine and I repeat my apologies.”
Both Ms Bercow and Lord McAlpine’s solicitor said the judgment would serve as a warning to social media users.
Ms Bercow’s tweet appeared two days after a BBC Newsnight report in November 2012 wrongly implicated the former Conservative treasurer in allegations of sex abuse at Bryn Estyn children’s home in the 1970s and 1980s.
She denied the tweet – “Why is Lord McAlpine trending? *Innocent face*” – was defamatory, but the peer’s lawyers said it pointed “the finger of blame” at him during a media frenzy.
Lord McAlpine, who has already received six-figure payouts from the BBC and ITV, said it meant he was a paedophile who was guilty of sexually abusing boys living in care.
The high court was told that Sally Bercow promptly tweeted her apologies, provided letters apologising for the distress caused and making clear that the allegations were untrue, and made an offer to settle the case which had not been withdrawn.
There will be another hearing at a later date on the appropriate level of damages, unless the two sides reach a settlement.
Other tweeters who have fallen foul of Lord McAlpine
George Monbiot: the Guardian columnist apologised and agreed to carry out £25,000 worth of charity work.
Alan Davies: the actor and comedian also apologised and could end up paying £200,000 to charity.
Others: Lord McAlpine suggested that tweeters with fewer than 500 followers should make modest donations to BBC Children In Need.
Mr Justice Tugendhat that in its natural and ordinary meaning, the tweet meant that Lord McAlpine was a paedophile who was guilty of sexually abusing boys.
He added that, if he was wrong about that, he would find that it bore an innuendo meaning to the same effect.
Lord McAlpine’s solicitor, Andrew Reid, said: “The apologies previously received from Mrs Bercow did not concede that her tweet was defamatory. Clearly she must now accept this fact.
“The failure of Mrs Bercow to admit that her tweet was defamatory caused considerable unnecessary pain and suffering to Lord McAlpine and his family over the past six months.
“With knowledge of the judgment, I am pleased to be able to say that Mrs Bercow has finally seen sense and has accepted an offer of settlement, which Lord McAlpine made back in January.”
Mr Reid said the judgment “provides both a warning to, and guidance for, people who use social media”.
Mrs Bercow said she had not tweeted “with malice” and had not intended to libel Lord McAlpine.
But she added: “Today’s ruling should be seen as a warning to all social media users.
“Things can be held to be seriously defamatory, even when you do not intend them to be defamatory and do not make any express accusation. On this, I have learned my own lesson the hard way.”