By Tony Barber in Brussels

The European Union’s plans to embrace new member-states from south-eastern Europe were thrown into question on Friday after President Nicolas Sarkozy of France said Ireland’s rejection of a landmark EU treaty had ruled out enlargement.

Referring to the 27-nation bloc’s Lisbon treaty, to which Irish voters said No in a referendum last week, Mr Sarkozy declared: “No Lisbon, no enlargement.”

He was speaking to reporters after the first day of an EU summit in Brussels dominated by the crisis into which the bloc has plunged as a result of the Irish vote.

The Lisbon treaty is designed to modernise the EU’s institutions and streamline its voting procedures to take account of the EU’s expansion to 27 member-states from 15 in 2004.

“I would find it very strange for a Europe of 27, which has trouble agreeing on workable institutions, to agree to add a 28th, a 29th, a 30th, a 31st, which would definitely make things worse,” Mr Sarkozy said.

His remarks cast doubt over how France would handle the enlargement dossier after it takes over the EU’s six-month rotating presidency from Slovenia on July 1.

Croatia hopes to complete its EU membership talks by the end of 2009 and join the bloc a year or two later.

Other countries that once formed part of the now defunct communist Yugoslav state, such as Bosnia-Herzegovina, Macedonia, Montenegro and Serbia, have what the EU refers to as “a European perspective” – that is, the possibility of eventual membership.

However, at present the only formal candidates for membership are Croatia, Macedonia and Turkey, and Mr Sarkozy has long made clear he does not think Turkey should become a full member.

Turkey’s entry talks are likely in any case to be informally frozen if the Turkish constitutional court shuts down the ruling Justice and Development party, re-elected with a large majority last year, and bans Prime Minister Recep Tayyip Erdogan from politics.

Senior EU officials confirmed that EU enlargement had emerged at the summit talks as one of the first casualties of the Irish referendum.

“Technically, the one thing is not connected with the other, but several delegations said that without the Lisbon treaty there’ll be no further enlargement. And unanimity among member-states is the rule, so it’s a real problem,” one official said.

EU leaders meanwhile agreed that they would review how to overcome the Irish rejection of the Lisbon treaty at their next summit in October.

Ireland is likely to be offered additional guarantees of its sovereignty in areas such as defence, family law and taxation.

But EU officials said it would be more difficult in legal terms to let Ireland keep its own European commissioner, since this would involve rewriting the EU’s existing rules.

Many countries are anxious to settle the Irish problem once and for all by next March, because European Parliament elections are scheduled for June 2009.

It will become increasingly urgent to know whether these are to be held under the rules of Lisbon or those of the EU’s Nice treaty, which came into force in 2003.