Sinn Fein has warned Mr David Trimble that his sanctions against its ministers are undermining attempts to resolve the issue of paramilitary disarmament.
The party's two Stormont ministers, Mr Martin McGuinness and Ms Bairbre de Brun, called on Mr Trimble to abide by a High Court judgment which said his ban on their attendance of the North-South Ministerial Council was unlawful.
In a letter to Mr Trimble, they claimed he was "continuing to act in clear breach" of the Belfast Agreement. They also stated: "You have also given notice in an interview with the BBC on March 11th that you intend to persist in acting both illegally and with impunity in respect of these matters.
"This is entirely in keeping with the wreckers' charter you put to the Ulster Unionist Council. We would also remind you of your obligation on the arms issue under the terms of the agreement.
"Much of the difficulties we have all encountered since Easter 1998 is in no small measure down to the fact that you have consistently sought to mislead popular opinion and especially popular Unionist opinion about the terms of the agreement on this matter.
"Under the terms of the agreement you have an obligation which you share collectively with the two governments and the other pro-agreement parties to use any influence you may have to resolve this issue. You are not doing this.
"On the contrary, your approach has not served to resolve the issue but to make its resolution all the more difficult." Mr Trimble introduced the sanctions at a meeting of his party's ruling UUC last October. It is understood he intends to challenge the High Court judgment.