Cardinal Keith O'Brien, the leader of the Catholic Church in Scotland, accused the Coalition of trying to "redefine reality" and claimed the proposals were a "grotesque subversion of a universally accepted human right".
Cardinal O'Brien said: "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 re-define 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?"
Margot James, the first openly lesbian Conservative MP, criticised the "apocalyptic language" used by the Cardinal and accused him of "scaremongering".
"I think it is a completely unacceptable way for a prelate to talk," she said. "I think that the Government is not trying to force Catholic churches to perform gay marriages at all. It is a purely civil matter."
Labour's Deputy Leader Harriet Harman said she hopes the comments would not end up "fuelling or legiti- mising prejudice". "We have had prejudice, discrimination and homophobia for hundreds of years, that doesn't make it right."