One senior EU ambassador said on Friday the chances of negotiations starting on time were only 50-50 because Austria was holding out for the 25-nation bloc to spell out an explicit alternative to full membership.
Chancellor Wolfgang Schuessel, whose party is battling to avert defeat in a regional election on Sunday, also wants the EU to open talks immediately with Austria's neighbour, Croatia, although the two candidacies are not formally linked.
Britain has called an emergency foreign ministers' meeting in Luxembourg on Sunday evening to try to overcome the final hurdle to launching the accession process with the overwhelmingly Muslim country of 72 million people.
British Europe Minister Douglas Alexander was guarded when asked whether he was confident talks would go ahead on schedule.
"There will be an intensive period of discussion both on Sunday and also on Monday and I believe that we will be able to move forward, although there is clearly a lot of work still to be done," he told BBC Radio.
Austria Chancellor Schuessel (L)
has objections to Turkish entry
Germany reminded its EU partners of their unanimous decision last December to begin accession talks with Turkey and urged Europe to meet its strategic responsibility and be fair.
"We think it is important that the EU sends a clear signal to Turkey," Foreign Ministry spokesman Jens Ploetner told a news conference in Berlin.
Turkish Prime Minister Rajab Tayyip Erdogan said on returning from a tour of Gulf states on Thursday evening he was still confident talks would go ahead on 3 October and played down the obstacle.
Austria blocked an EU agreement on the negotiating mandate at ambassador level on Thursday, but Erdogan forecast that ministers would clinch a deal on Monday morning.
"There will be an intensive period of discussion both on Sunday and also on Monday and I believe that we will be able to move forward"
UK Europe Minister
"After it reaches its decision on the morning of 3 October, we will continue on our path. It is their problem in a way. But I don't think they have a very serious problem," he said.
Turkish Foreign Minister Abdullah Gul sounded less certain, telling a hastily arranged news conference on Thursday that the problem was serious and talks might not start on Monday at all.
Austria's stance reflects widespread public opposition in western Europe to the prospect of Turkish membership, which is expected to take 10 to 15 years.
The senior EU ambassador, speaking on condition of anonymity, said if the Austrian objection could not be overcome, talks would not start because Turkey would not turn up.
"We are still assuming there will be a solution on Sunday, but I would only give it a 50-50 chance," he said.
Turkish Foreign Minister Abdullah
Gul (L) sounded less certain
"Austria is saying its position is not just tactical. So I don't see this as a lever for Croatia," the envoy added.
Other diplomats think a key may lie in the hands of the chief UN war crimes prosecutor, Carla del Ponte, who was in Croatia on Friday to discuss cooperation in the hunt for fugitive indicted ex-general Ante Gotovina.
If Del Ponte reports that Zagreb is now cooperating satisfactorily with her tribunal, the EU may be able to give a green light for Croatia's talks on Monday, allowing Austria to claim a victory that might enable it to back down over Turkey.
In Austria, political parties were united behind Schuessel's stance, except for the Greens.
Opposition Social Democratic leader Alfred Gusenbauer told Schuessel he must stick to his tough line after Sunday's regional polls in Styria, and should only allow Turkey's talks to go ahead if the EU agrees to offer a "privileged partnership" as an alternative to full membership.
Turkey has said it would walk away rather than accept such a second-class status.