Slovenian voters are casting ballots in a referendum that might turn it into the first Slavic, ex-Yugoslav and ex-socialist country to legalize gay marriage.

The poll was called by the Constitutional court at the request of a civic organization backed by the Catholic church, despite Parliament's move to legalize gay marriage in March, with a majority of 51 (and 28 against) backing the move.

Until now, civil unions were allowed in the country but partners were unable to adopt children.

Legislation passed in March, however, redefines marriage as "a union of two", replacing the previous concept of marriage as "a union of a man and a woman".

A similar referendum was held in 2012, when a narrow majority of 55% turned down the proposal. To reverse the legal changes now, a turnout of at least 20% and a "no" majority" of over 50% will be required.

Both Slovenia's President and the country's PM support the "yes" camp.