The Nice Treaty contained a serious flaw, two former European Commission officials have told the National Forum on Europe in Dublin Castle.
A provision of the treaty required that, when membership of the European Union reached 27, "the number of members of the Commission shall be less than the number of member-states".
In a joint submission Mr Eamonn Gallagher and Mr John Temple Lang said they supported enlargement of the EU. "But we do not accept that there is any justification for making the institutional balance of the union victim of enlargement."
The treaty provision in question is part of Article 4 of the Protocol on enlargement.
No information had been given as to what figure was in mind for the eventual size of the Commission but they believe "20 or even fewer" was the intention.
"This could leave as many as a quarter of the member-states without a nominee on future commissions when this clause becomes operative."
The Commission had been set up as an institution independent of national governments.
"Several member-states, including Ireland, expressed the view in the Nice Treaty preparations that to ensure a strong and effective Commission it was necessary to include a nominee from each member-state, including the newly-admitted candidate countries," they said.
A Commission that was not fully representative would have a diminished capacity to defend the interests of the EU as a whole, not to mention the interests of smaller member-states and minorities generally.