The No to Nice Campaign, whose leaders include the former judge, Mr Roderick O'Hanlon, is to seek an injunction in the High Court this morning to prevent the Taoiseach from repeating allegations that the campaign is funded by right-wing fundamentalists in the US.
Speaking at the weekend in Killarney, Co Kerry, Mr Ahern said that £100,000 was being pumped into what he called the "black-and-red poster campaign" based in his Dublin Central constituency.
The bulk of the money for this "sinister campaign of disinformation" came from a right-wing fundamentalist organisation in the US, he said.
An angry spokesman for the No to Nice campaign said the funds raised had come from Irish donors. The figure cited by the Taoiseach was "absolutely absurd". The Taoiseach had made "libellous" accusations and a court challenge would be made as quickly as possible.
The No to Nice group has an office in Capel Street, Dublin. A spokesman for the campaign said it would also be seeking costs from the Taoiseach for "malicious falsehood".
The campaign showed bank statements from the AIB branch in Upper O'Connell Street to The Irish Times last night. Out of a total £30,436 received in donations, only one American contribution of $125 was listed. There was a further contribution of 100 Canadian dollars and a handful of sterling cheques for modest amounts.
The bulk of the money was Irish, raised by advertising in the Irish Catholic and other religious publications, according to the group.
In Tralee, Co Kerry, yesterday, the Taoiseach described the leadership of the No campaign as "the Green Party, Sinn Fein, a range of small single issue groups, and assorted hardliners of the extreme right and extreme left". They were being cheered on by "the lunatic fringe" of the Tory party and a few Danish socialists, he said.
Mr Ahern was accused of indulging in "fantasies" about funding for the No campaign by the anti-Nice campaigner, Mr Anthony Coughlan, of the National Platform, who said his organisation had received about £700 following an appeal in the London-based European Journal.
The Socialist Party TD, Mr Joe Higgins, accused Mr Ahern of "smear tactics" in the campaign. "A note of panic is emerging in the Government camp regarding the outcome of the Nice Treaty referendum," Mr Higgins said.
The Fianna Fail group in the European Parliament said the No lobby offered no alternative. "In fact, they cannot point to one provision of the Nice Treaty that they would renegotiate if such an opportunity was afforded to them."
The Green Party TD, Mr Trevor Sargent, said in a statement that a No vote "simply means a pause until the treaty is amended. The treaty can be amended by adding an Irish protocol similar to the protocol negotiated by the Danish government. An improved treaty can then be put before the people before the December 2002 deadline for ratification".
The Labour MEP for Dublin, Mr Proinsias De Rossa, said the "widespread confusion" about the Nice Treaty acutely demonstrated the need for the Taoiseach to take up the Labour Party's proposal for a Forum on the Future of Europe.