The Italian Senate approved on Thursday a watered-down bill allowing civil unions for same-sex and heterosexual couples, with the government using a confidence motion to ram the contested legislation through the upper house.

To overcome opposition from within his own center-left coalition, Prime Minister Matteo Renzi had to strip out the most controversial part of the text, which would have granted unmarried couples some adoption rights.

The issue split parliament down religious lines, with politicians close to the Roman Catholic Church arguing that what was dubbed a 'stepchild adoption clause' would encourage gays to have babies with surrogate mothers, which is illegal in Italy.

The motion was approved by 173 to 71. If he had lost the vote, Renzi would have had to resign. The bill will pass to the lower house of parliament later this year for final approval.

Italy is the only major Western country that has yet to recognise civil unions and Renzi hailed Thursday's vote as historic, saying he had been right to put the government's future on the line over the heated issue.

"Hope has won against fear. Courage has won against discrimination. Love has won," he wrote on his Facebook page.

But gay rights organisations expressed bitter disappointment over the dropped adoption provision, which followed months of ferocious political debate and mass rallies across Italy.

"This text once again does not take into consideration children who need definite laws and protection," said Flavio Romani, president of gay rights group Arcigay. "The law that has come out of all this is lacking its heart."

Underscoring the chasm between the opposing lobbies, Interior Minister Angelino Alfano, who leads the small New Centre Right party (NCD), said Italy had "prevented a revolution that went against nature" by denying gays scope for adoption.

Renzi took office in 2014 and promised to enact the law last year but has faced more opposition over this particular reform than for any of his other initiatives, including a contested shake up of the labour market.

The bill gives homosexual couples the right to receive a deceased partner's pension, the right to take a partner's name, inheritance rights and next-of-kin rights in medical emergencies – all allowed only in marriage so far.

Heterosexual couples are granted next-of-kin rights, but the other provisions are only obtained through traditional marriage.

In another concession to Alfano's NCD party, the government dropped language that opponents said made the civil unions comparable with marriage. As a result, couples signing up to the partnership will not be required to promise to be faithful.