Croatia's top court on Wednesday suspended sexual education lessons in all public schools, arguing that the government failed to consult parents when it introduced the classes that are fiercly opposed by conservative groups and the Catholic Church.
"Enabling parents to take part in the preparation of a curriculum is a country's constitutional obligation," said judge Mato Arlovic.
Consulting pupils' parents was "notably important in a curriculum linked to different attitudes and beliefs," he added.
The suspension came after complaints from two Church-backed conservative associations, a right-wing party and a citizens' group, which claimed that the sex-ed programme violated the constitution because it was implemented without parental approval.
All sexual education classes will be on hold in state primary and high schools until the court rules on whether the lessons comply with Croatia's constitution.
Prime Minister Zoran Milanovic, whose centre-left government introduced the classes in February to raise awareness of sexual health, said the current curriculum was "in line with the constitution".
He said court rulings "have to be honoured" but insisted that sexual education should be part of a school curriculum.
"I will not give up in this fight," Milanovic said.
The sexual education lessons were to be taught to pupils aged nine to 18, officials said, for just three hours on average per school year.
The plan was to cover topics such as sexually transmitted diseases, masturbation and gender equality, with content adjusted to pupils' ages.
The course also sought to help prevent unwanted teenage pregnancies in a country that sees some 2,000 such cases annually, as well as around 400 abortions among adolescents each year.
But the classes ran into angry opposition from conservatives, including Church bishops, who argued that they would promote "pornography, promiscuity and homosexuality".
They insisted parents should be consulted in setting the curriculum.
Almost 90 percent of Croatia's population of 4.2 million are Roman Catholics and the Church has a strong influence in the country, which is set to join the European Unino in July.
Recently, a Church-backed group "In The Name Of The Family" launched a petition seeking a national referendum on whether to include a definition of marriage as a "union of a man and a woman" into the constitution.
Croatia's current constitution does not contain a provision defining marriage.