President Jacques Chirac of France was under pressure on Sunday night to get more involved in campaigning for a Yes vote in May's referendum on the European constitution as a second opinion poll in a week showed hardening public opposition to the treaty.

In a poll to be published in Le Figaro on Monday, 52 per cent of those who said they planned to vote in the referendum on May 29 are against the treaty, compared with 40 per cent in early March. The poll, conducted by Ipsos on Friday and Saturday, follows a poll by Le Parisien published on Friday showing for the first time a majority against the constitution.

A No vote by France to the treaty would almost certainly kill the constitution and plunge the EU into crisis. The treaty needs to be ratified by all 25 member states, and a French rejection would leave the EU facing the dilemma of how to move forward.

On Sunday party leaders called on Mr Chirac to increase his efforts to persuade voters to vote Yes. François Bayrou, the leader of the UDF, said yesterday: “Faced with a choice of this magnitude, the president is obliged to speak up.” François Hollande, the leader of the Socialist party, has also called on the president to make his views clearer.

The decline in support recorded in the Ipsos poll of 860 people was sharp among leftwing voters.

The president has remained largely distant from the campaign. On Friday he said he still believed France would back the charter. But it is clear that the constitution is becoming a focal point for general dissatisfaction with the government. The past month has seen a series of strikes and demonstrations against reform of the 35-hour week and an unemployment rate that remains stubbornly around 10 per cent. Some opposition to the treaty is also caught up with the debate on Turkey's eventual entry to the EU.

“Some will vote against the referendum for reasons that have nothing to do with the referendum,” said one senior government adviser. “It shows there is a growing gap between the establishment and the man and woman in the street. A No could increase the fragility of the country because of this gap.”

Since France is a founder member of the EU and the constitution was drafted by Valéry Giscard d'Estaing, a former president, a No vote would be also damning indictment of the growing distance between the EU and its citizens.

Without the constitution, the EU would be left with an outdated and complicated voting system and would be less able to act on the world stage: plans to create an EU foreign minister and full-time president would collapse.

The constitution also aims to involve national parliaments more in EU decision-making and to give the EU a greater role in the fields of asylum and immigration and judicial co-operation.