Italy's constitutional court has ruled against legislation that automatically gives children of married couples the father's surname.
Lawyers argued that preventing families from giving children their mother's surname discriminated against women.
The European Court of Human Rights (ECHR) had earlier condemned the legislation - which dates from Roman times - and ordered Italy to change it.
Campaigners hailed Tuesday's ruling and called for parliament to endorse it.
"The court has declared the unlawfulness of rules providing for the automatic attribution of the paternal surname to legitimate children, when the parents wish otherwise," the constitutional court said in a statement.
The case involved an Italian-Brazilian couple who wanted to give their son both their surnames, as is traditional in many Latin countries.
After their request was rejected by Italian authorities, they took the case to the ECHR, which ruled in their favour in 2014.
It said the law was incompatible with the principle of gender equality enshrined in Italy's modern constitution.
Italy's lower house has approved a bill aimed at changing the law, but it has been blocked in the Senate for years.
"The Constitutional Court has taken a decision of great importance for our society," campaigner and left-wing MP Fabrizia Giuliani is quoted as saying by AFP news agency.
"The senate no longer has any excuse for not abolishing this anachronism and giving women their right in this matter."