Sally Bercow is being sued for £50,000 by Lord McAlpine after she sent a tweet which the former Conservative chancellor alleges linked him with false child abuse claims.
The Guardian reports Bercow, who is married to the House of Commons' speaker, will fight Lord McAlpine at the High Court and has instructed London lawyers Carter-Ruck to defend her.
McAlpine's lawyers reportedly filed a libel claim last Friday after failing to settle.
Carter-Ruck confirmed to the Press Gazette that they would take on Bercow's case.
McAlpine took action after a BBC Newsnight report led to the peer being mistakenly implicated in a paedophile ring that targeted children at a care home in Wrexham in north Wales.
During the furore, Bercow tweeted: "Why is Lord McAlpine trending? *innocent face*"
She has said the tweet was not defamatory and later tweeted she was "VERY sorry for inadvertently fanning flames" about the prominent Conservative.
She later added in a tweet to her 57,111 followers: "I guess I'd better get some legal advice then. Still maintain was not a libellous tweet - just foolish."
Last month Lord McAlpine settled with the BBC and ITV over the false claims, for £185,000 and £125,000 respectively.
At the time, he and his lawyer said they would continue to seek damages from those who had linked his name with allegations of child abuse in North Wales. "We will now be continuing to seek settlements from other organisations that have published defamatory remarks and individuals who have used Twitter to defame me," McAlpine promised.
McAlpine has said the suggestion of being a paedophile was the worst thing of which anyone could be accused.
"I don't want to be too dramatic about the thing, but Boris (Johnson) got it right. There is nothing as bad as this that you can do to people," he said.
"Because they are quite rightly figures of public hatred. And suddenly to find yourself a figure of public hatred, unjustifiably, is terrifying."Suggest a correction