Amongst the signatories were 35 Tory MP and others including 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.”
Tory MPs will be given a free vote on the issue when the bill comes to parliament, and many suspect that the majority of “nay” votes will come from Tory backbenchers. Despite this the bill is expected to pass with high levels of support within the Labour and Liberal Democrat parties.