Croatia looked poised on Thursday to join Europe's 26-nation open travel zone, but Bulgaria and Romania appeared likely to be kept out because of Austrian concerns over growing unauthorised immigration.
The so-called Schengen area is the world's largest free travel zone, which abolishes border checks between member states in what is hailed as one of the top achievements of European integration.
But emergency controls have popped up regularly over the last decade during periods of increased immigration from the Middle East and Africa, and at times when member countries have struggled to contain local security threats effectively.
The EU's executive European Commission has already recommended that Bulgaria, Romania and Croatia should be admitted to Schengen, saying they had met the necessary technical requirements.
But accession needs unanimous backing from all members - the 22 EU nations along with Lichtenstein, Iceland, Norway and Switzerland.
But Austrian Interior Minister Gerhard Karner told reporters as he arrived at a key meeting in Brussels that he would vote against letting in Romania and Bulgaria.
"It is wrong that a system that does not work properly in many places would get expanded at this point," he said, adding that the admission of the two eastern EU states could pose security concerns.
He said Austria had recorded 100,000 illegal border crossings so far this year, including 75,000 people who had not been previously registered on the continent, something that should not happen in a country deep inside the Schengen zone.
'Hot button issue'
Immigration has been a hot button issue in Europe since 2015, with more than a million people arriving across the Mediterranean Sea, mostly on smugglers' boats, prompting the EU to tighten its external borders.
UN data shows some 145,000 people have made the sea crossing this year while more than 1,800 perished trying to reach Europe's shores.
The EU's border police, Frontex, said last month that 281,000 irregular entries had been recorded throughout the bloc in the first 10 months of 2022, up 77 percent from a year before and the highest figure since 2016.
With the Western Balkans route the most active currently and the EU welcoming several million Ukrainians fleeing the Russian offensive, worries about immigration have returned to the fore, complicating the case for Romania and Bulgaria.
The Netherlands has sided with Austria in opposing granting access to Bulgaria, citing concerns over corruption, as well as migration.