Surrogate parents should be treated as sex offenders and sent to prison, Italy's interior minister said on Wednesday, escalating his opposition to plans to give gay and unmarried heterosexual couples legal recognition.

Italy is the last major country in the West that has not given same-sex couples rights or protection on issues such as parenthood. It was condemned last year by the European Court of Human Rights for failing to legislate on the matter.

But minister Angelino Alfano, whose small New Centre Right party (NCD) opposes any form of surrogacy, is particularly incensed by a proposal to let gay partners adopt their stepchildren, saying this subverts traditional family values.

"Stepchild (adoption) really risks bringing the country closer to wombs-for-rent, towards the most vile, illegal trade that man has invented," Alfano told Avvenire newspaper, Italy's mainstream Roman Catholic newspaper.

"We want wombs-for-rent to become a universal crime, which is punished with a jail term. Just as happens for sex crimes," he added.

Centre-left Prime Minister Matteo Renzi had promised to legalise civil unions before the end of 2015, but the deadline slipped though coalition infighting which has highlighted the sway that Roman Catholic teaching still holds over Italy.

The contested bill is due to return to parliament on Jan. 26, with most of the tension focussed on whether unmarried partners should be allowed to adopt their stepchildren under certain circumstances.

Proponents of the plan say this would protect the rights of a child if its natural parent died. Opponents such as Alfano say it would open the way for gay couples to seek children via surrogate mothers.

Surrogacy is illegal in Italy and can lead to prison terms. Looking to circumvent the law, a number of couples have used surrogates abroad, but the status of their children is legally shaky and has led to prolonged battles in the Italian courts.

Some opposition parties, including former prime minister Silvio Berlusconi's Forza Italia (Go Italy!) party, have said they will support the bill, meaning that Renzi should be able to muster the numbers needed to carry it into law.

However, Alfano has warned the prime minister not to seek ad-hoc deals outside the walls of the ruling coalition, saying such a move could get out of control.

"(A law on) civil unions is not in the government programme. There is a risk that a snowball might become an avalanche," he told Rai state television.