Sally Bercow said the High Court's ruling that her tweet about Lord McAlpine was libellous "should be seen as a warning to all social media users".
Mrs Bercow said in a statement that she did not write the tweet "with malice" and "did not intend to libel" the Conservative peer.
I was being conversational and mischievous, as was so often my style on Twitter.
I very much regret my tweet, and I promptly apologised publicly and privately to Lord McAlpine for the distress I caused him. I also made two offers of compensation.
Lord McAlpine issued proceedings and the last few months have been a nightmare. I am sure he has found it as stressful as I have. Litigation is not a pleasant experience for anyone.
More top news
Australian businessman named as Alan Sugar's newest apprentice and winner of a £250,000 investment in his fledgling company.
The 270 victims of the Lockerbie bombing have been remembered today, 26 years to the day that they lost their lives in the air disaster.
Actress Billie Whitelaw, who starred in films including The Omen, has died aged 82.