France's standing in Europe could suffer if voters reject the EU constitution in May, French President Jacques Chirac has warned.
Chirac says France must back the treaty to retain its EU standing
Mr Chirac had been under pressure to come out in favour of the treaty as the No campaign grows more vocal in France, observers say.
Recent polls show the French would vote against the treaty in the 29 May vote.
A "No" vote in a large member state would throw the EU into crisis and leave it with antiquated institutions.
Polls in the last few days suggested a majority of French voters could oppose the EU's first-ever constitution.
"It is certain that if France blocked the European project... it would lose a large share of its authority, which is necessary, in the Europe of tomorrow," Mr Chirac told reporters in Brussels.
Mr Chirac was speaking at the end of an EU summit on European services.
"I believe this current constitutional treaty marks progress in social, economic and foreign matters," he added.
The text is designed to streamline institutions to make decision-making easier in an enlarged union of 25 countries.
On Tuesday, EU Commission chief Jose Manuel Barroso said France needed to explain the constitution to voters.