Sally Bercow, the wife of the Commons Speaker, is being sued for libel over a tweet which allegedly linked a former Conservative politician to false child abuse allegations.
Law firm Carter-Ruck confirmed it is representing the outspoken former Celebrity Big Brother contestant and ex-Labour council candidate against a £50,000 claim by Lord McAlpine.
The former Tory politician was mistakenly implicated by Newsnight's November 2 broadcast about a paedophile ring that targeted children at a care home in Wrexham.
The peer's name was then widely mentioned on the internet, including social networking site Twitter, by users including Mrs Bercow, wife of John Bercow, comedian Alan Davies and Guardian columnist George Monbiot.
A spokesman for Carter-Ruck confirmed it was representing the 43 year old, against proceedings of libel brought by Lord McAlpine.
"He's limited his claim to £50,000 in the court," the spokesman said.
Mrs Bercow deactivated her account but has now reopened it, sending a message on the site which said: "Humbly reactivating account on #Levesoneve *grabs tinsel & mince pies* coz learnt lesson, miss Twitter & will not screw up again. Hello!"
Although the programme did not name the peer - referring only to a senior Conservative from the Thatcher era - he was quickly identified online.
Newsnight carried a full, on-air apology for the broadcast a week later.
Lord McAlpine has asked those who linked him to child abuse allegations to apologise formally and pay a "sensible and modest amount", which he plans to donate to BBC Children In Need.
In a statement, Lord McAlpine's lawyer Andrew Reid said: "I am disappointed that the press has become aware of the proceedings between Lord McAlpine and Sally Bercow.
"However, I am still hopeful that we will be able to reach a sensible agreement to settle this matter with the second set of solicitors, instructed by Mrs Bercow, without the need to proceed to a trial.
"We have no further comment to make, and do not intend to deal with this issue through the media."