Voters were asked three questions including whether marriage can only be a union between a man and a woman.

Slovakia's population of 5.4 million people is predominantly Catholic.

Liberals and gay rights activists welcomed the outcome. They had encouraged people to boycott the referendum.

Over 90% of those who did vote backed the 2014 constitutional amendment which defines marriage as a union of a man and a woman, effectively banning same-sex unions and adoptions.

They also supported the right for children to skip classes involving sex education and euthanasia.

Anton Chromik of the Alliance for Family which had spearheaded the referendum said he was delighted that a clear majority of voters who had participated supported the Alliance.

"We will continue our struggle to protect the family," he said as the low turnout became apparent.

Conservatives fear that although the gay marriage ban remains in place, it is being undermined by liberal policies spreading eastwards from western Europe.

EU rules allow each of the bloc's 28 members to pass their own laws on issues like marriage and adoption.

While several west European countries have legalised same-sex unions, some eastern EU members have taken a more conservative line.

Croatia held a similar referendum in 2013, backing a proposal to ban gay marriage, while in neighbouring Hungary, Prime Minister Viktor Orban has championed traditional family values.