The appointment of a powerful head of the European Council will lead to creation of a new bureaucracy and damage the European Commission, the Taoiseach, Mr Ahern, has warned.
Highlighting Irish priorities for the remaining negotiations on the Convention on the Future of Europe, Mr Ahern said he was sceptical about the value of a council president.
However, he indicated that the Government might be willing to cede its right to hold the EU presidency for six months once a decade or so - if countries were put into teams to run presidencies.
"For example, Ireland would chair Ecofin, while Luxembourg might chair social affairs. I feel that this can be done," the Taoiseach told Mr Joe Higgins TD of the Socialist Party.
"If there is a president of the European Council, he or she will immediately set up an alternative bureaucracy. While this may not happen overnight, it will happen over a number of years. they will not use the Commission and there will be a clash between the president of the council and the president of the Commission. Both will play to the parliament and it will break what has been a successful position," he went on.
However, the Taoiseach opposed efforts to reopen debate about the size of the Commission once the EU reached 27 members, as agreed during difficult talks in Nice in 2001.
Then, EU leaders agreed that the biggest five countries would immediately lose their right to have two commissioners. In return, not every country would be represented on the Commission once the EU reached 27 members. "There will be strict rotation so that when Ireland has a commissioner, for example, Germany has not, or when Germany has a commissioner Ireland has not. At least it is a fair and balanced system. Some countries and the Commission itself are now arguing in favour of an alternative position. They would like to go back to having a commissioner per member state.
"The reason I did not support that at Nice was that I thought it would result in a two-tier or three-tier Commission arrangement with a front bench, second bench and third bench.
"This arrangement is on the table now, but my fear is that - not tomorrow but in three or four years time, there will be a hierarchy of commissioners," he told Sinn Féin Cavan-Monaghan TD, Mr Caoimhín Ó Caoláin.
In reply to Mr Ó Caoláin, the Taoiseach said he had "no such ambitions" to become president of the council. Under the convention's plan, the president would first have had to be a prime minister for years.
He said the next three weeks were "crucial" as the final draft of the convention's recommendations to the inter-governmental conference is teased out in advance of the Thessaloniki Summit in Greece in late June.
"Then we will see how much is in square brackets, how many options there are, and how it goes into the IGC. I might as well be totally honest about this as I can that it could come into the Irish presidency.
"I am endeavouring to be as helpful as I can to the Greeks and getting everyone I can to narrow down their positions so that we do not end up like the justice and home affairs area where there are 700 amendments to one section," he said.