Pressure was mounting on Monday ahead of a crunch EU summit this week.
Following pre-summit talks among EU foreign ministers, Italian Foreign Minister Franco Frattini rejected fierce demands by Poland and Spain on national voting rights that threaten to send the summit into overtime.
"The presidency won't accept a constitutional treaty that would take us backward," he told a news conference.
"We can't have compromises at the lowest common denominator. A bad constitution would be worse than no constitution at all," he said.
The EU heads of government will meet on Friday and Saturday for make-or-break talks that will decide success or failure for a two-year-long exercise in constitution making.
The 15-nation bloc's first-ever charter is meant to get the EU's creaking institutions in shape for its biggest enlargement yet, with 10 more countries due to join on May 1.
But Poland - the biggest of the 10 incoming members - and Spain are fighting to retain the generous voting power they secured in 2000 under the EU's Nice Treaty rather than accept a new arrangement foreseen in the draft constitution.
That would jettison the Nice voting weights, which gave Poland and Spain almost as much influence as Germany, and let a vote pass if it wins the support of a majority of countries representing 60 percent of the EU population.
But Frattini said the Italian presidency would leave the new arrangement proposed by an EU convention untouched, "because we don't feel that an alternative proposal has really come through that might enjoy success".
"A bad constitution is not an option"
German foreign minister
The Italians received strong backing from France and Germany, the traditional engines of European integration which favour the delicate compromises teased out over 17 months by the EU convention.
"We have to adopt clear decisions in Brussels. Everyone has to accept responsibility," French Foreign Minister Dominique de Villepin said, adding that nobody wanted "an agreement at any price".
Germany's Joschka Fischer said he was staving off pessimism about the outcome of the summit, but added, "A bad constitution is not an option.
"The challenge for the summit is whether it can cope with the task of enlargement, the reunification of Europe. That is the yardstick, not national interests, even if they are important," he said.
The EU foreign ministers also ran into difficulties over a "mutual defence" clause in the draft constitution that would require other member states to come to the defence of one under attack.
Austria, Ireland, Finland and Sweden, which are constitutionally neutral or non-aligned, resisted language that would force them to join any military action.
Frattini said their concerns would be taken on board, while British Foreign Secretary Jack Straw said there was no chance of reluctant EU countries being dragooned into a military campaign.
On another bone of contention, the EU must sort out who has the final say over the bloc's 100-billion-euro ($122-billion) annual budget.
"The European Parliament is a worthy body, but it does not sustain a government or raise taxes"
British foreign minister
The European Parliament is furious over calls by EU finance ministers to curb its powers of financial oversight in the new constitution.
But Straw said it was only right that member states have a more equal say in any disputes over the budget.
"The European Parliament is a worthy body, but it does not sustain a government or raise taxes," he said.