Writing in the Sunday Telegraph, the most senior Catholic in Scotland, Cardinal Keith Michael Patrick O’Brien, has said that gay marriage is a “grotesque subversion of a universally accepted human right”. He suggests that same sex marriage will lead to three way marriages and compares the government’s support for equality to legalising slavery.

The Conservative MP Margot James said Cardinal O’Brien’s language was “unacceptable” while the Home Office says that couples should have the right to marry “irrespective of their sexual orientation”.

His article comes just a week after the spiritual head of the Church of England, Rowan Williams said that the Anglican communion will fight gay marriage. Introducing gay marriage is supported across the political spectrum including by the SNP first minister of Scotland, Alex Salmond; the Conservative prime minister of the UK, David Cameron; the Liberal Democrat deputy prime minister of the UK, Nick Clegg; and by the Labour leader Ed Miliband.

In his article today, Cardinal O’Brien writes: “On the surface, the question of same-sex marriage may seem to be an innocuous one. Civil partnerships have been in place for several years now, allowing same-sex couples to register their relationship and enjoy a variety of legal protections. When these arrangements were introduced, supporters were at pains to point out that they didn’t want marriage, accepting that marriage had only ever meant the legal union of a man and a woman.

“Those of us who were not in favour of civil partnership, believing that such relationships are harmful to the physical, mental and spiritual wellbeing of those involved, warned that in time marriage would be demanded too. We were accused of scaremongering then, yet exactly such demands are upon us now.”

Mr O’Brien claims that gay marriage redefines marriage for everyone: “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.

“Redefining marriage will have huge implications for what is taught in our schools, and for wider society. It will redefine society since the institution of marriage is one of the fundamental building blocks of society. The repercussions of enacting same-sex marriage into law will be immense.

“But can we simply redefine terms at a whim? Can a word whose meaning has been clearly understood in every society throughout history suddenly be changed to mean something else?”

“In Article 16 of the Universal Declaration on Human Rights, marriage is defined as a relationship between men and women. But when our politicians suggest jettisoning the established understanding of marriage and subverting its meaning they aren’t derided.”

Controversially, Mr O’Brien writes that gay marriage is “madness and grotesque”: “Their attempt to redefine reality is given a polite hearing, their madness is indulged. Their proposal represents a grotesque subversion of a universally accepted human right.”

“There is no doubt that, as a society, we have become blasé about the importance of marriage as a stabilising influence and less inclined to prize it as a worthwhile institution.”

Mr O’Brien then refers to a constant theme of anti-gay marriage supporters, that the purpose of marriage is to procreate. “It has been damaged and undermined over the course of a generation, yet marriage has always existed in order to bring men and women together so that the children born of those unions will have a mother and a father.

“This brings us to the one perspective which seems to be completely lost or ignored: the point of view of the child. All children deserve to begin life with a mother and father; the evidence in favour of the stability and well-being which this provides is overwhelming and unequivocal. It cannot be provided by a same-sex couple, however well-intentioned they may be.

“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.”

Last month, a senior Church of England bishop, the bishop of Salisbury said that the argument that marriage was created as an institution to support the birth of children was a irrelevant and outdated. He told The Times: “Contraception created a barrier in that line of argument. Would you say that an infertile couple who were knowingly infertile when they got married, weren’t in a proper marriage? No you wouldn’t.”

Mr O’Brien also appears to suggest that gay marriage may lead to three way marriages: “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? If marriage is simply about adults who love each other, on what basis can three adults who love each other be prevented from marrying?”

He also claims that schools will become forced to stock “homosexual fairy stories” in their libraries.

The prime minister David Cameron promised that churches won’t be forced to conduct gay marriages but Mr O’Brien says the government is being disingenuous and “staggeringly arrogant.” He also compares gay marriage to legalising slavery.

“No Government has the moral authority to dismantle the universally understood meaning of marriage.

“Imagine for a moment that the Government had decided to legalise slavery but assured us that ‘no one will be forced to keep a slave.’ Would such worthless assurances calm our fury? Would they justify dismantling a fundamental human right? Or would they simply amount to weasel words masking a great wrong?”

A Home Office spokesman said the British 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”.

Margot James, openly lesbian Conservative MP said: “I think it is a completely unacceptable way for a prelate to talk. I think that the government is not trying to force Catholic churches to perform gay marriages at all. It is a purely civil matter.”

Liberal Democrat candidate for Mayor of London, Brian Paddick said: “Same-sex marriage should simply be a universally accepted human right for everyone. If we really believe in equality, there is no sound intellectual argument against gay marriage. There may be religious objections, as there are religious objections to equality for women, but that does not mean we should be ruled by them.”