The judgement was welcomed by campaigners as a milestone in a long legal and political battle to overturn regulations and practice they say are based on outdated patriarchal ideas.
“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 court said in a statement on Tuesday.
The court made its judgement in a case referred to it by a Genoa appeals court in which an Italian-Brazilian couple wanted to give their son both their surnames, as is traditional in Spain and much of South America and increasingly common among younger couples in northern Europe and North America.
The couple's lawyers had argued that not allowing the son to have his mother's surname, as well as his father's, violated the principle of equality between the sexes.
As the court did not publish its reasoning immediately, there was still a question mark as to whether the case had opened the door to Italian parents being automatically able to give their kids double surnames, or whether the equality requirement could be met by offering only the right to choose between the mother and father's surname.
Campaigners said the court's ruling showed the urgent need for clear legislation after decades of parliamentary stalling on reform.
Legislation allowing children to be given their mothers' surnames was first proposed 40 years ago. A draft bill has since been approved by the Chamber of Deputies but has been blocked for years in the Senate.
“The Constitutional Court has taken a decision of great importance for our society,” said Democratic Party deputy Fabrizia Giuliani.
“The Senate no longer has any excuse for not abolishing this anachronism and giving women their right in this matter.”
How quickly things will change remains to be seen. The constitutional court ruled in 2006 that the automatic attribution of paternal surnames was not consistent with the principle of equality between the sexes.
Italy's rules have also been condemned by the European Court of Human Rights, which ruled in 2014 that they could be used to discriminate against women.
Neither of those rulings led to reform of either the law or administrative practice.