Cardinal Keith O’Brien, the leader of the Catholic Church in Scotland, says the proposals to allow same-sex unions are “madness” and a “grotesque subversion of a universally accepted human right”.
The cardinal’s intervention, in an article for The Sunday Telegraph, is the strongest criticism yet from any church figure of the plans, which are due to be unveiled this month by Lynne Featherstone, the equalities minister.
He accuses ministers of trying to “redefine reality” and change long-standing laws and traditions “at the behest of a small minority of activists”.
The cardinal has added his voice to those of leading figures in the Coalition for Marriage, a group of bishops, politicians and lawyers opposed to the changes. The group’s supporters include Lord Carey, the former Archbishop of Canterbury.
The group is in outright opposition to Mr Cameron, who hopes to make legislation changing the legal definition of marriage to include same-sex couples, expected by 2015, one of the central achievements of his time in office.
Mrs Featherstone is to launch a consultation on how the changes will come about this month. Despite opposition from some sections of his party, the Prime Minister has personally associated himself with the proposals.
Mr Cameron told last year’s Tory conference in Manchester: “I don’t support gay marriage in spite of being a Conservative. I support gay marriage because I am a Conservative.” Last week his spokesman said he was “passionate” about the issue.
Cardinal O’Brien, the only British Catholic to be part of the College of Cardinals, the body which elects popes, accuses ministers of showing “intolerance” and coming up with plans that would “shame the United Kingdom in the eyes of the world”.
The new clash between the Coalition and the Catholic Church comes as the Church of England also appears increasingly split over the issue of gay rights.
A new proposal for a deal that would effectively prevent openly gay clergy from becoming bishops is on the brink of failure despite the personal endorsement of the Archbishop of Canterbury, Dr Rowan Williams.
The proposed agreement, known as the Anglican Covenant, has been rejected by 13 dioceses so far, with only eight backing it.
Defeat would, it was said, be a “devastating” blow to Dr Williams. The Covenant needs the approval of a majority of dioceses for a vote at the General Synod.
The Government, meanwhile, faces a Commons rebellion of scores of Conservative MPs over its gay marriage proposals. Some Tory ministers are among those opposed. Any vote is many months away, however, and Labour would be almost certain to back the move.
The legislation would apply only in England and Wales. However, Scottish MPs would be able to vote on the plans – a fact which has helped provoke Cardinal O’Brien’s intervention today.
He writes: “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.”
Ministers have assured church groups they will not be forced to accept same-sex marriages – a pledge which does not impress the cardinal. He writes: “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?”
Campaigners say the state has no right to change the definition of marriage according to most Christian thinking – a definition which is enshrined in laws dating back almost 800 years.
Mrs Featherstone, however, has argued that marriage is “owned by the people” and that governments have a duty to change laws to bolster the “underlying principles of family, society and personal freedoms”