The referendum proponents meanwhile said that they had collected in only four days more than 40,000 signatures needed to call the vote and announced they would challenge the National Assembly's decision at the Constitutional Court.

They believe that the bill passed by the MPs was illegal and anti-constitutional and that it violated rights guaranteed by the Constitution. The group also called a press conference for tomorrow.

The debate in parliament today revolved above all around the right to adopt, which homosexual couples would get if the changes enacting marriage equality will take effect after being passed by the National Assembly earlier this month.

Adoptions are the most controversial issue for referendum proponents, a conservative civil initiative, which enjoys the support of the Roman Catholic Church and the opposition Democrats (SDS) and New Slovenia (NSi) among others.

The Democrats (SDS) MP Branko Grims referred to a recent statement by gay fashion designers Domenico Dolce and Stefano Gabbana, who spoke out against gay adoptions.

The SDS also deems as problematic the fact that grandparents cannot adopt their grandchildren, while homosexuals could. "I could not adopt my granddaughter, but social services would have the right to give her to a gay couple," said Marijan Pojbič of SDS.

This argument, used also by the referendum proponents, has however been refuted by Labour, Family and Social Affairs Ministry officials explaining that grandparents cannot adopt because the procedure would cut off the child from the other side of the family.

The officials added that the social services had taken into account the wishes of the family and children were in fact very often placed with their grandparents, however not through the process of adoption but in foster care.

Marija Bačič of the Social Democrats (SD) on the other hand urged those campaigning against marriage equality to think about the benefits for the children and expressed belief that gay adoptions would not become a frequent occurrence.

Alenka Bratušek of the opposition Alenka Bratušek Alliance (ZaAB) accused referendum proponents of abusing children for political purposes and criticised the Catholic Church for supporting the referendum motion while failing to act in sexual abuse cases involving priests.

The opposition Pensioners' Party (DeSUS) MPs remain divided and voted according to their conscience. MP Jana Jenko said that today's session was important because it would give a chance to the Constitutional Court to say whether the referendum is admissible.