BRUSSELS - When European Union leaders sign a treaty to overhaul the bloc's outgrown institutions at a ceremony in Lisbon on Thursday, several of them may have their fingers crossed behind their backs.

With only Ireland planning a referendum on ratification, the prospect of another train wreck like the 2005 French and Dutch "No" votes to the proposed EU constitution may seem slight.

Yet polls suggest many Irish voters are undecided or indifferent, and parliamentary ratification in Eurosceptical countries such as Britain cannot be taken for granted.

Nevertheless, the leaders are reasonably confident the latest attempt to adapt the bloc's structures to its enlargement from 15 to 27 countries with about 500 million citizens will succeed after nearly a decade of failed reform efforts.

The treaty negotiated under German Chancellor Angela Merkel's stewardship and concluded under Portuguese presidency in October is meant to give the EU stronger leadership, a more effective foreign policy and more democratic decision-making.

"We cannot continue in this world without the treaty. The EU would lose a lot in Africa, in Asia, in world fora, at the United Nations," Slovenian Foreign Minister Dimitrij Rupel told Reuters in an interview.

His small ex-communist former Yugoslav republic, which joined the EU in 2004, takes over the bloc's much-criticised rotating presidency for six months in January.

The reform treaty would replace that unstable system of leadership in 2009 with a long-term president of the European Council to chair summits, and a stronger foreign policy chief in charge of a real diplomatic service and a big aid budget.

It will also allow more decisions to be taken by majority voting, notably on justice and security matters, and give more say to the European and national parliaments.

OPPONENTS WANT VOTE

French President Nicolas Sarkozy broke two years of EU political deadlock after the failed referendums by calling when he took office in May for a "simplified treaty" that could be ratified by his parliament.

But the content is so similar to the defunct constitution that opponents notably in Britain, who fear a creeping EU "superstate", have challenged its legitimacy and demanded a popular vote.

The British government negotiated complex exemptions from the Charter of Fundamental Rights in the treaty and opt-outs from the majority voting provisions on justice and home affairs.

Prime Minister Gordon Brown will miss Thursday's signing ceremony but attend the leaders' lunch and sign the treaty afterwards, his spokesman said.

Brown has to appear before a parliamentary committee that morning and British officials dismiss any suggestion that he does not want to be seen signing the text.

Danish Prime Minister Anders Fogh Rasmussen dashed the hopes of Eurosceptics on Tuesday by announcing his country would ratify the treaty in parliament, where a majority supports the charter.

The news is a relief to Brown, who has faced down calls from the Conservative and Liberal Democrat opposition and some in his Labour party for a plebiscite that polls suggest he would lose.

The signing ceremony will be held in the cloisters of Lisbon's Jeronimos Monastery, a symbol of Portugal's power in the 15th and early 16th centuries when it discovered the sea route to India.

Although the treaty omitted the trappings of statehood from the constitution such as the European flag and anthem, organisers said Beethoven's Ode to Joy would be sung live at the ceremony and the EU's gold-starred blue flag will be on display.