Family values campaigners have called on peers to defeat the plans.

Under current legislation, single gay men and lesbians can adopt - although all unmarried couples are precluded from doing so.

Opposition to any change in the law was being led by Conservative peer Lady O'Cathain, who is championing the family values cause following the death of Baroness Young earlier this year.

She warned that moves to allow unmarried and gay couples to adopt are a further attempt at "social engineering" and will undermine the institution of marriage.

Stable Family Life

The Tory peer argues that children are best served when brought up in a stable, married, family unit.

Campaigners claim that unmarried couples are nearly seven times more likely to break up.

In a letter to the Telegraph, senior Anglican, Roman Catholic and Muslim figures have urged peers to defeat the government's plans.

A change in the law would raise "deep questions of conscience" for religious people involved in the adoption process, the letter states.

The correspondence is signed by the Bishop of Winchester, the Rt Rev Michael Scott-Joynt, the Roman Catholic Archbishop of Cardiff, the Most Rev Peter Smith and the secretary general of the Muslim Council of Britain, Iqbal Sacranie.

The letter comes as The Christian Institute launches a new "adoption card" for parents against gay adoption.

"In the event of my death I do not want my children to be adopted by homosexuals," reads the card.

Human Rights

The government has defended its plans, insisting that increasing the pool of possible adoption households will ensure that fewer children remain in care.

Gay rights groups have called on peers to support the bill in the interests of human rights.

Angela Mason of the pressure group Stonewall said: "For us the bill is about whether children coming out of care have the extra security of having two parents rather than one.

"This is the issue, it is really quite wrong to say that a child adopted by a gay man or a lesbian should only have one parent."

Lady O'Cathain has dismissed accusations of homophobia. "I'm not gay bashing; I am pro-children," she said.

Conservative peers have been whipped to vote against attempts to change the law, although Labour and Lib Dems will be allowed a free vote on the issue.

Signs of a compromise grew yesterday when several peers signalled that they would support an amendment giving unmarried heterosexual couples new adoption rights.

Mason said such a move would be discriminatory towards gays and lesbians and argued it could be challenged under human rights legislation.