The Sinn Fein Minister for Health has begun legal proceedings to overturn the ban on her attending meetings of the North-South Ministerial Council.
Solicitors acting for the Department of Health lodged papers for a judicial review with the Belfast High Court late yesterday afternoon.
The proceedings arise from the refusal of the First Minister to nominate Ms Bairbre de Brun to attend a council meeting earlier this month.
Ms de Brun was instead forced to hold a bilateral meeting on health and food safety with her Southern counterpart, Mr Martin. The Deputy First Minister also attended the meeting but, as Mr Mallon and Mr Trimble nominate ministers jointly, both are cited in the papers.
Announcing her challenge Ms de Brun said: "The action of the First Minister is restricting my ability to carry out my duties as a Minister.
"His action is both discriminatory and anti-democratic. He is acting with impunity on this matter," she said.
"I have a responsibility to challenge any action which seeks to prevent me from fulfilling my responsibilities as a Minister. I fully intend to do so."
Ms de Brun said Mr Trimble was in breach of his pledge of office as First Minister, the ministerial code and the Belfast Agreement. She criticised the Northern Secretary for not compelling Mr Trimble to nominate Sinn Fein ministers to the council.
Ms de Brun said Mr Mandelson had this power under the 1998 Northern Ireland Act but it appeared he was not prepared to exercise these powers. Legal sources say the Department of Health's papers could be before a judge as soon as Monday with Mr Trimble being required to lodge submissions in the following days.
The Sinn Fein chairman, Mr Mitchel McLaughlin, said the party might still decide to take its own action. "We had come to the view that we should launch a legal action. But Bairbre de Brun's announcement this morning will have to now be factored into our planning and our thinking on this matter," he said.
Ulster Unionists hit back at Ms de Brun for using taxpayers' money to fund the legal challenge.
Mr Alan McFarland, a member of the Assembly's Health Committee, said the case was unwinnable and accused the Minister of wasting time and public money.
"We all know at the current time there are people on trolleys all over the province. Indeed, one of my constituents last week had to go to Craigavon to get a hospital bed," he said.
"If we are saying that the Minister is now using precious public money from the Health purse to pursue a vendetta against the Ulster Unionist Party, then it is crazy," he said.
Mr Nigel Dodds, a Democratic Unionist Assembly member, said the legal action against Mr Trimble showed just how "farcical and incoherent this form of government in Northern Ireland has become".
It is unclear how long the legal process might take. One republican source said it could last up to two weeks.
Meanwhile Ministers from Sinn Fein, the Ulster Unionist Party and the SDLP held their regular Executive meeting. Sources described the atmosphere as "businesslike".
The Executive also considered a consultation paper produced by the Ministerial Group on Public Health and agreed to its publication at the end of the month.