Family members cast ballots at a polling station during a referendum on whether to give same-sex couples the right to marry and to adopt children, in Sora, Slovenia December 20, 2015. REUTERS/Srdjan Zivulovic

Slovenians have rejected a same-sex marriage law by a large margin, according to preliminary referendum results.

The results released on Sunday by authorities show 63% voted against a bill that defines marriage as a union of two adults while 37% were in favour.

The results are still incomplete but are unlikely to change significantly in the final tally.

Parliament introduced marriage equality in March but conservative groups, backed by the Catholic Church, have pushed through a popular vote on the issue.

PA Media