The head of the board responsible for the new national children's hospital says it cannot be certain of the final price of the project.
The project has been highly controversial, this week the Labour party claimed the project would now exceed €2bn.
A letter from David Gunning, the chief officer of the National Paediatric Hospital Development Board, read out at the Public Accounts Committee today said: "There have been a number of exceptions that have been outside the approved budget for which there cannot be price certainty at this point or for the duration of the project."
Speaking at the committee earlier, Chairman Sean Fleming described it as an "extraordinary admission" and that the committee "has to get to get to the bottom of it".
Meanwhile, Labour's Alan Kelly said "We see an escalation in costs at a very significant scale in the space of a few months.
"The public is so conditioned to this now that in any other situation there would be a national outcry.
"We now have a contract for construction and then we have also in a very convenient way for the Government a separate process which cannot be costed on an ongoing basis and will only be revealed when the whole thing is over in years to come.
"That cannot happen. The public deserves to know on an ongoing basis all costs related to this. I would now ask the Taoiseach to guarantee that this project isn't going to go over €2bn.
"This thing is now out of control- we don't know where this is going to end. "
Fianna Fáil's Marc McSharry added that the project "effectively amounts to a blank cheque and who knows where its going to end up. "
Social Democrats' Catherine Murphy said: "One of the things that was flagged early on is that wage inflation may well be a reason why there's an even further overrun but this is not monopoly money and you would have to have serious concerns about the cost.
"There's a degree of spin around this and we have to keep a very close eye on this. We need to have somebody come in to us and talk to us about the cost control side of this."
Members of the National Paediatric Hospital Development Board are due to appear again before the Public Accounts Committee in a few weeks time.