LJUBLJANA (Reuters) - The Slovenian Supreme Court on Wednesday annulled the result of a September referendum that approved the government’s biggest investment project and ordered a new referendum on the project.

In September, 53.5 percent of voters backed the construction of the 27-kilometre (17-mile) railway line between the city of Divaca and the state-owned Adriatic seaport Luka Koper, a project worth 1 billion euros ($1.24 billion).

But Vili Kovacic, head of the civil society group Taxpayers Don’t Give Up, complained to the court, saying the government had an advantage in the referendum because it used 95,000 euros of budget funds to support its campaign.

“The government’s campaign was inadmissibly one-sided,” the court said and ordered the State Election Commission to set a new referendum date. The Commission is expected to set the date in the coming days.

The referendum was initiated by Kovacic’s group, which argued that the line was too expensive and asked voters to reject the project.

The center-left government has so far received 44.3 million euros of European Union funding for track preparation, and neighboring Hungary has said it was willing to invest about 200 million euros in the project, since it relies on the port of Koper for much of its seaborne freight.

The court’s decision adds to the troubles of the government of Prime Minister Miro Cerar three months before a parliamentary election, which is expected on June 10.

Also on Wednesday, teachers demanding higher wages held their second one-day strike this year, closing most schools and kindergartens. Other public-sector unions also threaten strikes unless the government agrees to hefty wage hikes.