Plans for a referendum on abortion could be scuppered as it falls into a "legal quagmire", the Dáil heard today.
Fine Gael and the Labour Party said the Bill, which should provide for the public poll by next spring, was not constitutional.
But Mr Ahern said it had been "well and truly checked" and would pose no legal difficulties.
In the referendum the public would have the chance to vote to make abortion illegal in all cases unless the mother's life was directly at risk.
The need for clarification came out of a 1992 High Court ruling in which it was deemed that an abortion could be carried out where the mother threatened suicide.
But that reason would be ruled out under the constitutional amendment bid.
In addition, the Bill would impose a 12-year prison penalty on anyone carrying out an illegal abortion.
This meant that there were two "separate and distinct" sections to the Bill, said Labour Party leader Mr Ruairi Quinn.
"This appears on our analysis, and on the legal advice available to the Labour Party, to be in breach of the procedures for amending the constitution," he said.
"The article that allows for the amendment of the constitution says that you cannot piggyback on to a proposal to amend the constitution. You can have a series of proposals to amend the constitution, but you cannot have a proposal to amend and, in addition, have another proposal which in this instance is the criminal statute piece of legislation."
He added: "I wonder if you could ensure the house that we are not embarking upon another legal quagmire that will raise more constitutional problems than it will solve."
Fine Gael leader Mr Michael Noonan, shared Mr Quinn's fears, saying: "I think we are well down the road to a legal quagmire."
Mr Ahern rejected their attacks, saying any legal problems in the amendment would be "teased out" in the normal process of passing law.
"My advice, and I assure you it has been well and truly checked out, is that this does not in any way go against the constitution," he said.
"If there is any valid point to be made of course it will be looked at. These issues will be clarified.
"They will be treated in the way that Governments deal with these things in the time honoured way," he added.
Mr Quinn called for the legal advice that Mr Ahern has been given to be presented to the Dáil.