Fine Gael leader Enda Kenny told reporters at the National Ploughing Championships that he did not believe Mr Ahern's evidence to the Mahon tribunal.
Mr Kenny said he had given Taoiseach Bertie Ahern "his opportunity to explain his side of the story. Prior to the election we had the Bertie Ahern account of the events. Four days after the election the tribunal announced a set of circumstances which were completely different".
Asked if he believed what Mr Ahern had told the tribunal, Mr Kenny answered "No"! "Since you are asking that fundamental question, I do not believe him," he said.
"The Taoiseach has now given four days evidence at the Mahon tribunal and that evidence was given on oath to tell the truth, the whole truth and nothing but the truth," he said.
He said he had always said that every man should have his say and Mr Ahern has had his say at this point. "I am now assessing his answers, his testimony. I will discuss this with my front bench in the morning and make my decision and judgment at that point . . . what we are talking about here is the prime minister of our country where we know from four different versions of events was in receipt of €300,000 or equivalent money in four different accounts over four years.That is a situation which is very grave and very serious."
Mr Kenny said he had wanted the Taoiseach to go into the tribunal and answer questions clearly and unambiguously. "I have not heard a credible account of the circumstances surrounding those serious cash lodgements and payments."
He was not surprised by senior Ministers' support for the Taoiseach. "In the case of Fianna Fáil, they not do resignations. They never do . . . You can expect every Minister to put up a blanket defence of the Taoiseach."
Tánaiste and Minister for Finance Brian Cowen said it had been clear for some days the Opposition was going to make a call on Bertie Ahern to resign as Taoiseach, but there was no question of this happening.
"The Taoiseach has obviously no intention of resigning. The Taoiseach has obtained a mandate two or three short months ago to lead this Government in its third term. That's what he'll be doing. He has the support of his colleagues in so doing and we have to get on with running the country."
Mr Cowen said Mr Ahern would have the full support of the entire Government including the Greens and PDs.
Mr Ahern had given his recollections to the best of his ability on events that had happened 12 or 14 years ago and been cross-examined and answered questions on it, and it was up to the tribunal to adjudicate on it after hearing all witnesses, Mr Cowen said.
"I think the view being taken by Opposition politicians is for opportunistic reasons," he said.
"There's been no allegation put to the Taoiseach during his four days in public session about any allegation of corruption or improper dealing on his part, so I think that's an important point to make, that's why the tribunal was set up," said Mr Cowen.
"As far as he wants, for as long as he wants, he has our support."
Minister for Agriculture Mary Coughlan said putting a vote of no confidence was a matter for the Labour Party but the tribunal had to be allowed complete its investigations.
"I think its important to say that Justice Mahon did indicate that the Taoiseach was more than facilitating with anything he was requested to do," she said.
She said the fact Mr Ahern was before the tribunal did not reflect on him in any way and it should not be forgotten that the inquiry was set up to investigate planning matters by the Oireachtas.
Asked about the substantial sums in cash Mr Ahern was transporting, Ms Coughlan said that this was not related to the issues of bribery and corruption that the Mahon tribunal was investigating.
"I think it's a bit inappropriate and disingenuous when we are actually talking about a particular time in a man's life which many people can empathise with, in other words a marital breakdown and the way in which someone deals with that," she said.