Dozens of MPs and peers have signed up to a cross-party alliance in opposition to same-sex marriage plans claiming the government has no mandate to introduce legislation.
In total, 58 parliamentarians - including 35 Tory MPs - have put their names to an open letter to the Daily Telegraph saying the state should not be able to redefine the meaning of marriage.
Among the signatories are former Conservative leadership challenger David Davis, ex-ministers Tim Loughton, Sir Gerald Howarth and Sir Jim Paice, and the former Archbishop of Canterbury, Lord Carey of Clifton.
The letter says: "At the last election, none of the three main parties stood on a platform to redefine marriage. It was not contained in any of their manifestos, nor did it feature in the coalition's programme for government.
"We understand some parliamentarians support freedom for same-sex couples to marry, but we support a freedom from the state being able to redefine the meaning of marriage."
Although Tory MPs have been promised a free vote on the issue, there is deep anger among traditionalists in the party who say it is out of step with the instincts of their natural supporters and is driving away the activists they need to campaign for then.
Announcing the measures in the Commons last week, Culture Secretary Maria Miller sought to reassure opponents with a "quadruple legal lock" guaranteeing that no religious organisation or minister will be compelled to conduct a gay wedding agains their wishes.
However many remain unconvinced, fearing it would be open to challenge in the European Court of Human Rights.
Despite the entrenched opposition of some Tories, the legislation is expected to pass with overwhelming support from Labour and the Liberal Democrats.
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