With Austria stubbornly refusing to give the green light for the historic negotiations, one senior EU diplomat said there was no more than a 50 per cent chance of them starting on Monday as scheduled.
The deadlock is the biggest challenge yet faced in Britain's presidency of the EU.
The UK strongly backs moves to start negotiating with Ankara, which has been knocking on Europe's door for four decades, and all EU governments last year agreed to go ahead with talks. But since then referendum "no" votes on the constitution in France and the Netherlands have hardened opinion against Turkish accession.
EU foreign ministers will hold an emergency session to try to break the deadlock tomorrow evening. But so unpredictable is the outcome that the UK presidency has rescheduled the planned start of talks with Turkey for late afternoon on Monday so that the Turkish Foreign Minister need not leave Ankara until he knows the negotiations will definitely go ahead.
Yesterday, Tony Blair told Turkey's Hurriyet newspaper: "I sincerely believe that EU membership is Turkey's future. We shall work towards achieving that. How quickly Turkey's EU train reaches its destination will depend on how fast Ankara can make changes."
Most diplomats have assumed that, since it is isolated, Vienna will back down. But one EU diplomat said: "We are still assuming there will be a solution on Sunday, but I would only give it a 50-50 chance."
In Austria, which has regional elections tomorrow, none of the main political parties backs Turkish membership of the EU, and the Austrian Chancellor, Wolfgang Schüssel, has enjoyed a positive domestic press for his stance.
Ursula Plassnik, Austria's Foreign Minister, said her concerns as to "whether the European Union is ready and capable to accept Turkey as a full member, whether all implications of full membership have been sufficiently examined", are shared "all over Europe". She said that Austria is proposing "an option in case membership does not work out", adding that full membership is possible "one day - if Turkey fulfils the requirements and if the European Union is also in a position to absorb Turkey".
With its mainly Muslim population of 70 million, many of whom live in relative poverty, integration of Turkey within the EU would pose an unparalleled task. Its supporters argue that to reject Turkey would send a negative message to the Muslim world. Austria wants to change the language of the negotiating mandate for the talks and, according to diplomats, to remove a reference to accession as the objective. Its preference is to make it clear that the talks could lead to a lesser form of "privileged partnership".
Ankara has made it clear it will walk away from the negotiations if the possibility of second-class status is included in the text. Already the mandate for talks, expected to last a decade, are the toughest ever laid down for an enlargement and stress that negotiation are "open-ended".
Austria, meanwhile, is pressing for the start of EU membership talks with Croatia. These have been put on hold pending a verdict on whether Zagreb has achieved full co-operation with the UN war crimes tribunal in The Hague.
Carla Del Ponte, its chief prosecutor, arrived in Croatia yesterday to renew pressure on authorities to arrest and extradite a top war crimes suspect, Ante Gotovina, who she says is hiding in the country. She will make a recommendation to the EU after her visit, and is likely to conclude that co-operation is improving.
Many diplomats believe Austrian opposition on Turkey could be bought off by a giving Croatia a date to start talks, although that idea was played down yesterday by the UK presidency.Reuse content