LONDON (Reuters) - The head of the Roman Catholic Church in England will be the latest church leader to try to ambush the prime minister's attempt to legalise same-sex marriage when he launches his "no" campaign from the pulpit this weekend.
Archbishop of Westminster Vincent Nichols has written a pastoral letter to be read out during Mass in dozens of churches on Sunday, warning about the dangers of changing the legal definition of marriage.
Prime Minister David Cameron is already facing a religious backlash from many in the Anglican mother church, the Church of England, which has sometimes been called the "Conservative Party at prayer".
The head of the Church of England, Archbishop of Canterbury Rowan Williams, however, has said a new law for gay couples would amount to forcing unwanted change on the rest of the nation.
The argument echoes others elsewhere in Europe and beyond.
On Friday, Pope Benedict denounced the "powerful political and cultural currents" seeking to legalise gay marriage in the United States, where Maryland has just become the eighth state to allow it.
The British government is planning this month to launch a formal consultation document on allowing homosexual couples to marry, spearheaded by a minister from the Liberal Democrats, the junior partner in the government coalition. Equalities minister Lynne Featherstone argues churches do not "own" marriage law.
Cameron, who has compared his faith to that of a weak radio signal - "it comes and goes", supported the change during a speech last October.
Nichols' letter, which has been released in advance, says that "neither the church nor the state has the power to change this fundamental understanding of marriage itself".
"Changing the legal definition of marriage would be a profoundly radical step," it says.
"A change in the law would gradually and inevitably transform society's understanding of the purpose of marriage.
"It would reduce it just to the commitment of the two people involved. There would be no recognition of the complementarity of male and female, or that marriage is intended for the procreation and education of children."
The Roman Catholic Church, which has some 1.3 billion members worldwide, teaches that while homosexual tendencies are not sinful, homosexual acts are, and that children should grow up in a traditional family with a mother and father.
Gay marriage is legal in a number of European countries, including Spain and the Netherlands.
An online survey by Catholic Voices, a group created to put the Roman Catholic church's case to the British media, said seven out of 10 Britons believed that marriage should continue to be defined as a lifelong union between a man and a woman.
Last weekend, the most senior Roman Catholic cleric in Britain, Cardinal Keith O'Brien, said the plans were a "grotesque subversion of a universally accepted human right".
The Church of England is the established church in a country is an increasingly secular society.
One liberal bishop in the Church of England has even said he regards some same-sex couples he knows as being in a marriage.
The National Secular Society argues the issue is broader than gay marriage, warning that if the Catholic church wins this argument it will lead to "other regressive demands" such as a change to the abortion law.
"This is the Catholic Church trying to bully its way into political power through lobbying by intimidation and emotional blackmail," it said on its website.
Additional reporting by Philip Pullella in Rome; Editing by Robin Pomeroy