A High Court judge has ruled that a tweet written by Sally Bercow about Lord McAlpine was libellous.
The tweet written by Mrs Bercow, wife of Commons Speaker John Bercow, followed a report falsely implicating the Tory peer in allegations of child sex abuse.
The judge, Mr Justice Tugendhat, said Mrs Bercow's followers Twitter - who numbered 56,000 - would likely know by the time of her post the elements of the story told on Newsnight.
He told the court:
The judge said the reader would reasonably infer that Mrs Bercow had provided "the last piece in the jigsaw".
ITV News correspondent Nina Nannar reports:
Following the ruling, Mrs Bercow said she was "surprised and disappointed" by the decision, said she would "accept the ruling as the end of the matter".
She said the ruling "should be seen as a warning to all social media users".
Lord McAlpine's lawyer 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".