LJUBLJANA (Reuters) – Slovenia’s ruling coalition parties have joined the opposition in calling for a national referendum on a border pact agreement with Croatia in a move that could once again complicate Zagreb’s bid to join the European Union.

The deal, agreed by the Slovenian and Croatian prime ministers Borut Pahor and Jadranka Kosor and signed last week, appeared to boost Croatia’s chances because EU member Slovenia had blocked the country’s bid for almost a year over the dispute.

A referendum, which looks certain, given the support for it from opposition and ruling parties as well as Pahor and President Danilo Turk, could put the issue back on the table.

The four ruling parties – Pahor’s Social Democrats, the center-left Zares, the pensioners’ party Desus and the Liberal Democracy – said yesterday they would ratify the agreement only if it won majority support in the referendum, which is expected in early 2010.

A recent opinion poll showed 48.5 percent of citizens would support the agreement, while 14.6 percent remained undecided. However, in some earlier polls, a majority opposed the border accord.

If support for the agreement falls short of a majority in the referendum, this would call into question the legitimacy of the government, since it has made solving the dispute one of its main foreign policy goals.

“If the agreement fails to get majority support, Slovenia will first have to deal with the consequences for the present government,while it is also possible that Slovenia could again block Croatia’s EU talks,” said Borut Hocevar, an editor at the newspaper Zurnal24.

Under the agreement signed last Wednesday, an international arbitration team will settle a dispute over the sea and land border dating from the disintegration of Yugoslavia in 1991 and its ruling will be binding on both countries.

The four members of the center-left coalition government said they expected the accord would be approved.

“We are very confident that, after a public debate, most citizens will support the arbitration agreement,” Miran Potrc, an MP of the Social Democrats, told a news conference.