Britain's most senior Roman Catholic cleric blasted Sunday the government's plan to permit civil gay marriage, calling it "madness" and a "grotesque subversion".
Cardinal Keith O'Brien, leader of the Catholic Church in Scotland, added his voice to religious opposition to the proposals, ahead of the launch this month of a formal consultation on same-sex civil marriages.
Conservative Prime Minister David Cameron's government has drawn up plans to introduce homosexual civil marriages before the next general election in 2015.
O'Brien said that when politicians subvert the meaning of marriage, "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," he wrote in The Sunday Telegraph newspaper.
If the government attempts to go ahead with the plans, "their intolerance will shame the United Kingdom in the eyes of the world."
Civil partnerships for same-sex couples were introduced in Britain in December 2005, giving them similar rights to married heterosexual couples. However, the partnerships cannot legally be referred to as marriages.
"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," O'Brien wrote.
"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.
"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?"
The British government's Scottish Secretary Michael Moore told BBC television the plans did not involve changing religious marriage or imposing it on religious groups.
"What we are saying is that where a couple love each other and they wish to commit to each other for their life then they should be able to have a civil marriage irrespective of their sexual orientation," he said.
Archbishop of York John Sentamu, the second most senior cleric in the Church of England, Britain's biggest denomination, also condemned the plans back in January.
New laws allowing same-sex civil partnership ceremonies to be conducted in places of worship in England and Wales came into force in December, though no religious group is obliged to host them.