The enlargement of the passport-free Schengen Area to take in Romania and Bulgaria has been put in serious doubt ahead of an expected vote on Thursday.
Austria and Netherlands continue to have concerns about admitting one or both of the countries.
The opposition has been brewing for days but became patently clear on Wednesday after EU ambassadors met to discuss the topic.
Interior ministers from the European Union are expected to decide on the long-stalled candidacies today. A unanimous endorsement is required to admit new members into Schengen and abolish checks at all internal borders.
"Austria is the main hold-out," a senior EU diplomat said on Wednesday afternoon, speaking on condition of anonymity due to the sensitivity of the issue.
"It's already clear that there is no unanimity," said another diplomat. "Those who don't agree will make their points of view heard during the debate."
Austria argues the new influx of asylum seekers through the Western Balkan route demonstrates that Schengen's eastward expansion should be postponed.
The country is expected to receive more than 95,000 asylum requests this year.
"We are under enormous pressure from irregular migration, even though we are a landlocked EU country and not an external border country," Austrian Chancellor Karl Nehammer said last month.
"The European asylum system has failed."
For its part, the Netherlands is open to Romania's accession but remains opposed to Bulgaria's entry over what they say are unaddressed rule-of-law elements.
Bulgaria has had a caretaker government since August after a series of inconclusive elections.
However, both Austria and the Netherlands intend to support the third outstanding candidate to join Schengen: Croatia, making its admission a
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