Former Archbishop of Canterbury Lord Carey is lending his weight to a campaign against 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 public consultation on how to make civil marriage available to same-sex couples is to be launched next month.
Grassroots organisation Coalition for Marriage is launching a campaign against the moves, in a bid to halt attempts to redefine marriage.
The proposals were announced by 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.
Lord Carey wrote that he was "baffled" by Mr Cameron's statement at last year's Conservative Party conference, in which the Prime Minister said: "I don't support gay marriage in spite of being a Conservative. I support gay marriage because I am a Conservative."
In his article, Lord Carey said: "Like many others, I was baffled by this statement. Not because I begrudge rights and benefits to homosexual couples."
He said he was baffled "because this Government's proposal constitutes one of the greatest political power grabs in history."
He went on: "The state does not 'own' the institution of marriage. Nor does 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."
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."