The leader of the Catholic Church in Scotland has defended comparing the introduction of gay marriage to the legalisation of slavery.
Cardinal O'Brien said on Monday that countries that had brought in same sex marriage were "violating human rights" as it conflicted with Article 16 of the United Nations Universal Declaration of Human Rights.
The prime minister is a "passionate" advocate of the change, telling his party two years ago he supported gay marriage "because I am a Conservative".
Expectations are that the coalition will hold a free vote in the Commons at some point in the next year or so to allow same-sex couples to marry.
Cardinal O'Brien wrote: "Since all the legal rights of marriage are already available to homosexual couples, it is clear that this proposal is not about rights, but rather is an attempt to redefine marriage for the whole of society at the behest of a small minority of activists.
"Same-sex marriage would eliminate entirely in law the basic idea of a mother and a father for every child. It would create a society which deliberately chooses to deprive a child of either a mother or a father.
"Other dangers exist. If marriage can be redefined so that it no longer means a man and a woman but two men or two women, why stop there? Why not allow three men or a woman and two men to constitute a marriage, if they pledge their fidelity to one another?"
He also compared the plans to introduce gay marriage to "legalising slavery", a charge he defended in an appearance on BBC Radio 4's Today programme on Monday morning.
"It is a perfectly good example as to what could happen in our own country if we go this way," he said. "It's time to call a halt now to what you may call progress in society."
Cardinal O'Brien said that gay marriage would lead to "further aberrations" and warned "society would be degenerating even further than it has already degenerated into immorality".
"Those countries where it is legal are violating human rights. We know what the UN declaration states. We know what follows on from this. It seems to me to be the thin edge of the wedge and it's changing the whole notion of what marriage is and what a family is."
Earlier this week the Home Office defended the plans after Tory MP Peter Bone called them "completely nuts".
A Home Office spokeswoman said the Government believed that "if a couple love each other" and want to commit to a life together they should "have the option of a civil marriage irrespective of their sexual orientation".
Equalities minister Lynne Featherstone is due to launch a consultation on the plans later this this month.
In an editorial published on Monday the Daily Telegraph said the existing situation was "a classic but effective British compromise" and warned gay marriage may be too controversial.
While The Times has taken the opposite view and has come out in favour of the change. "Allowing same-sex couples to marry would enrich the institution and expand the sum of human happiness," the paper said.
Cardinal O'Brien's appearance on the radio caused a storm on Twitter, with several MPs attacking him for his comments.
Conservative MP Daniel Byles said the Cardinal was was "shaming himself" and Tory Gavin Barwell said he should "attack violence, theft & hate speech not people of the same sex who love each other".
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