LUXEMBOURG - European Union partners voiced concern on Saturday over growing opposition in France to the EU's first constitution, stressing there was no fallback plan if ratification is rejected in a May referendum.

Founding heavyweight France is the second EU state after Spain to put the bloc's first charter to a popular vote, on May 29, and an opinion poll published on Saturday showed 56 percent of French voters plan to say "No".

Senior EU leaders said that such a political earthquake would plunge the 25-nation bloc into uncertainty. Fear of such an outcome prompted EU foreign ministers, meeting in Luxembourg, to rally behind their French colleague, Michel Barnier.

"If France votes no, it will be difficult in Europe," Luxembourg Foreign Minister Jean Asselborn told reporters. "I am convinced the French will make the right choice."

Luxembourg holds the EU's rotating presidency until the end of June and would have to cope with the fallout of a negative vote in France.

Poland said a French "No" would not only spark a crisis, but could split the EU and throw integration into reverse.

"The consequences of not ratifying the European constitution will be a crisis and the possibility of divisions within the Union. It will create concepts of a two- or more-tiered Europe," Polish Prime Minister Marek Belka said in Warsaw.

Slovenian Foreign Minister Dimitrij Rupel echoed this sentiment in Luxembourg, saying: "I am extremely worried... Politically this would be a disaster."

French Government Confident

Barnier voiced confidence that the bad mood in France could be turned around before polling day, but a live television campaign debate with French President Jacques Chirac on Friday failed to convince voters.

"The referendum is not tomorrow. It's on May 29," Barnier told reporters. "I'm confident that we can explain by the 29th what's at stake."

The French debate is being watched closely by EU partners such as the Netherlands and Denmark which will also hold referendums on the treaty which aims to streamline decision-making after the EU's expansion to 25 members.

Denmark will vote on Sept. 27 and Dutch Foreign Minister Bernard Bot, whose country will vote just three days after France, said he was in close contact with Barnier about the campaign.

"We talk about it all the time in a very friendly fashion," Bot told Reuters. "He (Barnier) is very optimistic because he is constantly campaigning."

He stressed the EU had no contingency plan in the event French voters rejected the constitution.

"We have no plan B. You cannot have a plan B. It is "Yes" and that's the only way to discuss this item, so we go 100 percent for that outcome," he said.

Despite opinion polls showing that almost two thirds of the Dutch planned not to vote on June 1, Bot voiced optimism that the government would win the non-binding referendum. But he also said the bad mood in France was rubbing off on Dutch voters.

"I don't believe in panic and we shouldn't pull out all the stops all of a sudden," he told reporters.

The Netherlands, like France one of the founding members of the EU, has seen its strong support for the EU wane.

French voters are dismayed with the government led by Conservative Prime Minister Jean-Pierre Raffarin and the EU's push for market liberalization in Europe.

Dutch voters are concerned with the country's sluggish economy, immigration, high net payments to the EU budget and the EU's decision to open membership talks with Turkey.

Additional reporting by Jon Boyle in Paris and Niclas Mika in Amsterdam