David Cameron faces fresh pressure from church leaders over the Government's plans for gay marriage. Cardinal Keith O'Brien, the leader of the Catholic Church in Scotland, claimed the proposals were "madness".
The Prime Minister is a "passionate" advocate of the change, telling his party two years ago he supported gay marriage "because I am a Conservative".
Expectations are that the coalition will hold a free vote in the Commons at some point in the next year or so to allow same-sex couples to marry.
Cardinal O'Brien wrote: "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.
"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?"
Earlier this week the Home Office defended the plans after Tory MP Peter Bone called them "completely nuts".
A Home Office spokeswoman said the 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".
Equalities minister Lynne Featherstone is due to launch a consultation on the plans later this this month.