Fine Gael leader Enda Kenny has called on the Taoiseach to fully explain the payment he received for a public speaking engagement in Manchester in 1994.
Mr Ahern faced questions in the Dáil about the controversy today but Mr Kenny said he failed to clear up the issue which emerged for the first time yesterday and is being inquired into by the Mahon Tribunal.
Mr Kenny said: "His speech was about the Irish economy and he answered questions about the Irish economy. In return, the Taoiseach was given 8,000 (he did not make it clear if this was sterling or euro - if it was punts, then this would equate to a gift or fee of €10,000). It appears that the Taoiseach was invited to that function because he was Minister for Finance.
"I am aware of no occasion when a minister of any government was paid to speak at a function."
The Fine Gael leader said it appeared Mr Ahern had accepted €10,000 a "nixer" and lodged it to his bank account. The issue is "very serious and requires further investigation and explanation" Mr Kenny said.
Ministers are precluded from accepting gifts of over IR£500 (€650) in value under the Ethics in Public Office Act.
The Cabinet Handbookwhich explains ministers' responsibilities in such matters says: "Where such a gift is made to an office holder, by virtue of his or her office, it may be retained in the minister's department until its future is determined, whereupon it will be vested in the Minister for Finance.
"Any such gift, or where doubt exists about the value, should be notified to the Secretary General to the Government as soon as possible."
Mr Kenny said the payment Mr Ahern received for the Manchester engagement "clearly constituted a gift".
"If issues like this continue to arise regarding the Taoiseach's actions his position could become untenable," Mr Kenny said.
Mr Ahern revealed he had been paid for an engagement in Manchester during his interview with RTE television news yesterday.
Mr Ahern's language was unclear during interview but he said the engagement was not political. "I was talking about the Irish economy, I was explaining about Irish economy matters and I'd say there was about 25 people at that. The organisers of it, I spent about 4 hours with them ... I got 8,000 on that," Mr Ahern said.