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
The soap star, who is now an anti-bullying ambassador, said she was bullied so badly at school she developed an eating disorder.
Man United have confirmed the signing of France under-21 forward Anthony Martial on a four-year deal from Monaco
Ecuadorian officials considered a number of plans to smuggle the Wikileaks editor-in-chief out of their embassy in London.