The Taoiseach's failure to question Limerick West TD Mr Michael Collins months ago after learning that he had been fined for tax evasion has been strongly questioned.
As the Dáil returns today, the Opposition will concentrate on discrepancies between the accounts offered about Mr Collins's case by Mr Ahern on Friday and again yesterday.
Fianna Fáil's own internal inquiry, which could recommend to the party's National Executive that Mr Collins be told to quit his Dáil seat, will begin this morning.
Fianna Fáil last night said Mr Collins had told the Taoiseach in June that he would be named as a tax defaulter in a Revenue Commissioners' list.
However, the party spokesman said Mr Collins had also made it clear that the issue had "been sorted" and that he now had a tax clearance certificate.
Mr Ahern appears not to have asked, or to have been told about the detail of Mr Collins' problems, particularly whether or not he was a bogus non-resident account holder.
Questioned on Friday in New York, Mr Ahern said he had believed that all Fianna Fáil TDs had had a tax clearance certificate last year from the Revenue.
He then said he had "quite frankly" no knowledge of the Collins case and gave no hint that he had been first notified of the impending crisis in June.
Yesterday, however, the Taoiseach acknowledged that he had known about Mr Collins's tax difficulties, but he insisted that he had not known the details.
The Taoiseach's failure to question Mr Collins properly and in detail about his tax difficulties caused bemusement in Leinster House last night.
Mr Ahern's failure, or reluctance to do so, held little obvious advantage for Fianna Fáil and, indeed, made it impossible for the party to manage the current public relations crisis.
Instead, Mr Ahern appears to have done nothing. He failed to notify the Tánaiste, Ms Harney, who, though irked, is keen not to allow the controversy further to weaken the Government.
"I am not going to build this up into a big issue. It is not something that affects a Minister, or a Minister of State. If it were, I would certainly have expected to have been told as quickly as the Taoiseach became aware of it," Ms Harney said.
Meanwhile, The Irish Times understands that the Revenue Commissioners did not notify Mr Collins that he was suspected of holding such an account until January this year.
The Irish Times last week reported that Mr Collins had notified the FF Parliamentary Party chairman, Mr Seamus Kirk, in June, but Mr Kirk, too, did not seek full details.
However, a succession of FF colleagues have privately made it clear that they had been told all or most of the details by Mr Collins before the Dáil summer recess.
The FF internal inquiry, headed by Mr Kirk, will also include the party's general secretary, Mr Seán Dorgan, barrister Mr Colm Maguire, TDs Mr Seán Power and Mr Seán Fleming.
The Taoiseach's handling of the matter "further erodes his credibility to govern", the Fine Gael leader, Mr Enda Kenny, said last night.
"Why did Deputy Collins's tax problems not set off alarm bells in the Taoiseach's mind?" Mr Kenny asked.
The Fine Gael leader also questioned why Fianna Fáil had not given the Standards in Public Office Commission advance warning that there were difficulties ahead.
The Labour Party leader, Mr Pat Rabbitte, pointed to the similarities in the Taoiseach's handling of the Collins case with that of then Kerry South TD Mr Denis Foley in 2000.
Then, Mr Ahern had known for weeks Mr Foley was included in a list of Ansbacher account holders held by the Moriarty Tribunal, but he allowed Mr Foley to stay on as a vice-chairman of the Dáil's Public Accounts Committee.
"On that occasion too, the Taoiseach was only stirred into belated action when the matter became public," said Mr Rabbitte, who derided Mr Collins's "virtually meaningless" resignation of the party whip.
Mr Collins, who resigned the FF whip on Saturday, once more insisted yesterday in a brief written statement that he had complied with ethics rules.
Under these regulations, TDs must produce tax clearance certificates within nine months and sworn statements that they are up to date with taxes within one month of election.
