A two-speed Europe would create division and difficulties and is not the way to make progress on a treaty, the Taoiseach, Mr Ahern, said today.
Contradicting European Commissioner Mr Romano Prodi's recent comments that a two-tier system would be "desirable" if agreement on a new constitution could not be made this year, the Taoiseach said the EU must work collectively and stick together to reach consensus.
"There is the danger that if we cannot make sufficient progress some countries will become frustrated and want to move forward," he admitted.
"But my view is that we have to find compromises and move on together. There is an enhanced co-operation mechanism providing countries set the agenda together for everybody."
Speaking on the BBC1's Breakfast with Frosthe added: "A two-speed Europe that would see one group going ahead and others being left behind would create a lot of division and a lot of difficulties.
"There are integrationists in Europe that want to move at an enormously fast pace, there are others who want to move at a more moderate pace and it is the challenge for Europe to be able to deal with those issues in a sensible way."
Reaching consensus on a treaty is one of the key priorities of Ireland's EU presidency. Mr Ahern said that his Government's task was now to operate a period of reflection and to work out how to get discussions moving again.
One of the issues which instigated the collapse of talks last month was that of majority voting which put France and Germany at loggerheads with Spain and some of the countries joining the EU this year.
Mr Ahern said the split must be addressed.
"If it turns into big and small, if it becomes a group of the bigger countries or the original countries, or the larger group of the smaller countries joining now, it will be very hard to get consensus," he said.
"The best way that Europe operates in moving together and that requires us to deal with the voting issue that really pitches the Poles and Spain against France and Germany.
"It is an issue for everybody to collectively find a way of resolving." Speaking about the Belfast Agreement he said he was hopeful that he could work together with the Democratic Unionist Party, Sinn Fein and the British government to implement the planned review.
"We have to look at the changes which are required by the Agreement to make a partnership Government work.
"It is a major challenge but one that we have to confront and one that Prime Minister Blair and I are determined to do," he said.