Pope Francis issued a stern reminder of the Catholic Church's opposition to gay marriage on Friday as a fierce debate raged in Italy ahead of a vote that would give legal recognition to homosexual couples.

Next week, the Senate is due to resume debating a bill that would legalise civil partnership for homosexuals as well as for unmarried heterosexual couples. Many opponents say the law is a Trojan horse that would lead to legalising gay marriage.

The Argentine pontiff, who is perceived to have taken a more liberal line on gender issues than his predecessors, told Vatican judges "there can be no confusion between the family God wants and any other type of union".

"The family, founded on indissoluble matrimony that unites and allows procreation, is part of God's dream and that of his Church for the salvation of humanity," he said in an address to members of the Vatican court that rules on marriage annulments.

Despite the European Court of Human Rights condemnation of Italy last year for failing to introduce a law on civil partnerships, its passage has been held up by objections from politicians of all stripes.

Opposition parties and even some members of Prime Minister Matteo Renzi's centre-left Democratic Party (PD) are incensed by a proposal in the law that would allow homosexuals to adopt the children their partners had from previous heterosexual unions.

The presence of the Vatican in Rome is often cited as a reason Italy is one of the last major countries in the West not to give same-sex couples rights or protection on issues like parenthood.

The bill's author, PD Senator Monica Cirinna, told reporters this week that the ruling party was in a state of "high fever" as the vote approaches and the Church's position was always lurking in the background of the debate.

There has always been a clash between the non-religious and the Catholic members of the party," said Cirinna. "The great dome sometimes casts a shadow," she said, referring to St. Peter's Basilica.

Some fear the bill would open the way to loosening laws on surrogate motherhood, which is illegal in Italy. Interior Minister Angelino Alfano has said that those who break the law should be treated as sex offenders and sent to prison.

The conservative Northern League demonstrated at the Pantheon in Rome this week collecting signatures for their bid to remove the stepchild adoption provision from the bill ahead of the January 28 vote in the upper house.