The government launched a public consultation process on Thursday on its plan to let gay couples marry, giving Christians and others three months to air their strong objections while insisting it would not back down.
Clerics, including the Church of England headed by the Queen and the Roman Catholic hierarchy, say Prime Minister David Cameron is interfering in religion with a plan to extend the full legal status of marriage to homosexuals, who have since 2005 been able to contract unions known as civil partnerships.
Launching the consultation process, during which ministers will review suggestions from the public, Equalities Minister Lynne Featherstone said: The essential question is not whether we are going to introduce same-sex civil marriage but how.
Interviewed in the Independent newspaper, she made clear the plan to introduce legislation before the next election in 2015 is unchanged: There is no rolling back whatsoever, she said.
Although churches and other religions will not be obliged to modify their own marriage rules, about twice as many Britons now marry in secular, civil ceremonies than in religious rites.
The proposed changes would open civil marriage ceremonies to gay couples and also let those already in civil partnerships convert their status to that of married. And the new law would allow people stay married if one legally changed their gender.
Christian leaders say that flies in the face of the traditional view of marriage as being between a man and a woman.
Some in Cameron's own Conservative party are also dismayed by his commitment to gay marriage, and opponents have also complained that the government is not listening to them.
Colin Hart, campaign director for Coalition for Marriage, said: I always thought that a consultation was about listening to people and asking them their views, before making a decision.
Not only are they redefining the meaning of marriage, they're redefining the meaning of consultation.
His lobby group said almost 210,000 people had signed an online petition against same-sex marriage, with signatories including religious leaders and about a dozen lawmakers from across the political spectrum.
Featherstone said: I believe that if a couple love each other and want to commit to a life together, they should have the option of a civil marriage, whatever their gender.
The Church of England, an established church which forms part of the structure of the state, said: Arguments that suggest religious marriage is separate and different from civil marriage, and will not be affected by the proposed redefinition, misunderstand the legal nature of marriage in this country.
On Sunday, Archbishop Vincent Nichols, the most senior Roman Catholic cleric in England, wrote a pastoral letter to be read at mass across the country warning about the proposed changes:
There would be no recognition of the complementarity of male and female, or that marriage is intended for the procreation and education of children, he said.
A number of other countries have introduced marriage provisions for same-sex couples including Spain, Canada, Argentina and South Africa, the British government noted.
Although Britain is an increasingly secular society, with only a small minority regularly taking part in religious rites, the Conservative party has long been a supporter of traditional marriage, and has mooted giving tax breaks to married couples.
But Cameron himself, who became Britain's youngest premier in almost 200 years when he won power two years ago aged 43, has spoken out strongly in favour of the proposed reforms.
(Editing by Alastair Macdonald)