ROME - A controversial referendum to overturn a restrictive Italian law on fertility treatments and stem cell research failed Monday because an insufficient number of registered voters cast ballots.

Italy's Roman Catholic bishops, with the support of Pope Benedict XVI, had waged a campaign to persuade voters to stay away and keep voter turnout low.

It remained unclear what effect the church's call had on the turnout; many political analysts also cited a growing fatigue among voters over referendums in recent years.

For a referendum to be valid, more than 50 percent of eligible voters must cast ballots. But in voting Sunday and Monday, only 25.9 percent of those eligible went to the polls. The turnout was so low that both sides of the divisive issue expressed surprise.

Referendum proponents had sought to abrogate four sections of Law 40, which gives an embryo the same judicial status as a person. It prohibits sperm and egg donations, the freezing of embryos, the implantation of more than three fertilized embryos, the medical screening of those embryos prior to implantation or research using human embryos.

The restrictions have led many childless Italian couples to seek fertility treatment in other countries.

Cardinal Camillo Ruini, who heads the Italian bishops conference, had publicly urged Italians to stay home to protect the existing law. Monday, he said in an interview on the state-run RAI television that victory was a "a mistaken term" for the outcome.

"I only tried to do my duty as a bishop and listen to my conscience as a man, Christian and citizen," he said.

Emma Bonino, a leader of the Radical Party that sponsored the referendum, said the defeat was "bitter" but that opponents of Law 40 would "roll up our shirt sleeves to get this law changed."

As the date of the referendum approached, debate intensified on television talk shows and in newspapers and created divisions within political blocs. While the center-right majority led by Prime Minister Silvio Berlusconi in large part supported the abstention campaign, a few mavericks broke ranks. Deputy Prime Minister Gianfranco Fini, leader of the conservative National Alliance party, upset his colleagues by declaring he would vote for the measure. Some political figures called for his resignation as party leader.

Berlusconi, who did not vote, said he had not declared his intentions before the balloting "so as not to divide the country."