His warning came as leaders of 28 European countries came together in Rome to negotiate an EU constitution that would prevent decision-making gridlock once the bloc expands next year.
"We can make improvements but we should not modify the balance reached by the Convention," Valery Giscard d'Estaing, a former French president who led the 16-month-long convention, told Italy's Corriere della Sera newspaper.
"That would reduce the possibility of ratification in one or more member states, because consensus would be ruined," he added.
"The intergovernmental conference will examine the draft but I don't see the use in going back on amendments which were presented at the Convention. If an amendment has already been presented to the Convention, can one be adopted tomorrow with unanimity?"
Giscard d'Estaing was not invited to the extraordinary EU summit in Rome of leaders of the 15 EU states and the 10 future members.
The current Italian EU presidency hopes that the talks will result in a new Treaty of Rome, echoing the 1957 text which founded the European Economic Community, the EU's forerunner.
Italian Prime Minister Silvio Berlusconi greeted the leaders on the steps of Mussolini's grandiose marbled Palazzo dei Congressi in the fascist-era suburb of Eur, as a helicopter hovered overhead as part of draconian security measures. Berlusconi told the opening session that the birth of the European Economic Community in Rome in 1957 had been an act of faith.
"I don't think it's an exaggeration to say that today is a truly historic event," he said. "Today we need more than an act of faith, we need an act of willpower."
Valery Giscard d'Estaing (L) celebrates the first draft
Clear disagreement remains on key issues. Smaller countries notably fear moves to bolster the dominance of the bigger EU states, while eurosceptics are resisting attempts to take power away from national governments.
The talks are meant to end in December. If successful, they will result in a constitution to govern the EU after the 15-nation bloc admits 10 new members next May. Candidates Romania, Bulgaria and Turkey were attending as observers.