The former Archbishop of Canterbury Lord Carey has voiced his opposition to plans to legalise same-sex marriages.
In an article for the Daily Mail, Lord Carey claimed the proposal to change the status of marriage "constitutes one of the greatest political power grabs in history".
In September, it was announced that the coalition government would legislate for same-sex marriages by 2015, a proposal that Carey repeatedly describes as "baffling."
Carey argues that he was particularly confused by David Cameron's statement at last year's Conservative party conference, in which he said
"I don't support gay marriage in spite of being a Conservative.
"I support gay marriage because I am a Conservative."
A public consultation on how to make civil marriage available to same-sex couples is to be launched next month.
Carey writes that the Conservative policy is not about "conserving" anything, and that the national "bedrock" of marriage was neither owned by the state, or the church.
"The honourable estate of matrimony precedes both the state and the church, and neither of these institutions have the right to redefine it in such a fundamental way."
Grassroots organisation Coalition for Marriage is today launching a campaign against the moves, in a bid to halt attempts to redefine marriage.
The proposals were announced by the Lib Dem Equalities Minister Lynne Featherstone last year, but senior Tories made clear they had the strong backing of David Cameron - despite the likely opposition of many Conservative traditionalists.
Gay rights campaigner Peter Tatchell said: "The Coalition for Marriage is intolerant and out of touch. Its support for the ban on gay marriage is homophobic and discrimination.
"Coalition members are entitled to believe that same-sex marriages are wrong, but they are not entitled to demand that their opposition to such marriages should be imposed on the rest of society and enforced by law."