John Hemming, the Liberal Democrat MP who made a name for himself by saying things in Parliament which could not legally be said anywhere else, has been accused of being unfit for public office.

A fellow MP has written to the Speaker saying that Mr Hemming should be made to quit Parliament after a woman whose cause he championed was accused by a judge of being a liar. John Mann, Labour MP for Bassetlaw, accused Mr Hemming of being "foolhardy and irresponsible" and of having a "macho attitude" towards the courts.

Vicky Haigh, a horse-racing trainer and jockey, has been involved in a dispute with her former partner and with Doncaster Council over custody of her daughter. Mr Hemming took up her case after she attended a public meeting which he chaired in a room in the House of Commons, where she spoke about the case despite a court order banning publicity.

After the meeting, Ms Haigh received a court summons that appeared to threaten her with prison. Mr Hemming raised the case on the floor of the House of Commons, as a potential breach of parliamentary privilege and a threat to free speech. Mr Hemming also named Ms Haigh, which he was able to do because he was an MP speaking in the Commons.

Previously, he used parliamentary privilege to reveal that Sir Fred Goodwin, former head of the Royal Bank of Scotland, had taken out a super-injunction to prevent details of his affair with a work colleague from being published, under which it was forbidden to name Mr Goodwin or describe him as a banker.

Mr Hemming again used parliamentary privilege to expose Ryan Giggs as the footballer who had obtained a similar injunction to protect him from being named as having had an affair with the Big Brother contestant Imogen Thomas.

The Haigh case is different, because it involved a dispute over a child's welfare. All the details except the name of the seven-year-old child involved, known as X, are now public; a court hearing this week culminated in a woman who had acted as an investigator for Ms Haigh being jailed for nine months for contempt of court. Elizabeth Watson was accused of sending "aggressive and intimidating" emails and internet postings about social workers involved in the case.

Sir Nicholas Wall, the UK's leading family court judge, said Ms Haigh had accused her former partner, David Tune, of being a paedophile, knowing that it was a lie, and of behaving as if she thought she was above the law.

"Allegations of sexual abuse were first made by the mother and not by X. These were false and the mother knew them to be false. X was coached by the mother to make allegations of sexual abuse against the father," he said.

This judgment prompted Mr Mann, who was Vicky Haigh's local MP at the time, to denounce Mr Hemming for drawing attention to the case. "A gung-ho attitude to the breaching of court injunctions on the floor of the House is foolhardy and irresponsible," he said.

He added that Mr Hemming "clearly has a psychological obsession with the breaking of court injunctions and is not fit to be an MP".

Mr Hemming stressed yesterday that he had never commented on the rights or wrongs of the family court judgment, but raised the case because of the implications for free speech.

He told The Independent: "I think people have a right to talk to parliamentarians and I think Parliament should stand up for that. What I said in the House of Commons was about Doncaster Council's attempt to jail her, because once judges or councils start trying to lock people up in secret, I get very edgy. Apparently John Mann would like to see them locked up."