Brussels - The European Union debate over how to convince Irish voters to back the Lisbon Treaty stalled on Thursday as EU leaders at a summit in Brussels hit deadlock over a question of legal form, EU diplomats said.

EU leaders had been scheduled to decide on how best to soothe Irish voters' fears that the treaty would take away their country's neutrality and control of its tax and family laws within an hour of the summit's opening.

But after two hours of bilateral talks, they failed to reach agreement, pushing a general debate back to Friday, diplomatic sources said.

Irish voters rejected the Lisbon treaty, which is supposed to make the EU more efficient, at a referendum in June 2008.

Post-mortem analyses showed that Irish voters rejected the document out of fears that the treaty would end Ireland's neutrality and take away its control of tax law, family law and labour and social guarantees.

At a summit in December, EU leaders promised to give Ireland "legal guarantees" that the treaty would not affect those issues.

But controversy remains over the question of how the guarantees should be passed into law.

Ireland wants EU leaders to back the document as a legally-binding "decision of the EU", and then to ratify it as a protocol to the EU's founding treaties, giving it fundamental legal weight.

But other EU member states, especially Britain, say they do not want to have to ratify the document, since this could re-open debate on the deeply unpopular Lisbon treaty among their own citizens.

EU leaders are now set to discuss the issue through the night, before debating it again on the second day of the summit on Friday.(dpa)