A Conservative MEP for the South West has warned David Cameron that legalising same-sex marriage will prompt a grassroots party revolt.
Giles Chichester, who represents the Tory Party in Brussels, has written to the Prime Minister urging him to reconsider the "bizarre" move.
Mr Chichester, the party's energy spokesman in the European Parliament, believes it is a "mystery" why Mr Cameron is pushing the "militant gay agenda" championed by pressure groups.
While supportive of same-sex civil partnerships, he warns the proposals, launched this month, are among a series of policies working to "undermine the traditional family".
He writes: "Should this measure go through, it will cause many Conservatives to question their loyalty to a party which is no longer supporting values inherent to the party."
A 12-week consultation on allowing gay couples in England and Wales to marry is ongoing. Mr Chichester's intervention, similar to those made by backbench Conservative MPs, could prove difficult for the party's leadership, anxious to modernise its image.
The MEP was also critical of the failure to introduce a tax break for married couples and hit out at scrapping child benefit for some families.
In the letter, Mr Chichester says: "The worst way in which you are attacking the traditional family is through your bizarre attempt to redefine marriage.
"Why you have chosen to push the Stonewall [campaign group] militant gay agenda is a mystery to me because same-sex marriage was not in your manifesto and so far as I can tell from friends in the gay community, there is no majority within that minority in favour of trying to make same-sex unions or partnerships the same as traditional marriage. Equal yes, same no."
In his speech to the autumn Conservative Party conference in October, the Prime Minister announced that he supported same-sex marriage "because I am a Conservative". Mr Cameron added: "To anyone who has reservations, I say, yes, it's about equality, but it's also about something else: commitment.
"Conservatives believe in the ties that bind us; that society is stronger when we make vows to each other and support each other. So I don't support gay marriage despite being a Conservative. I support gay marriage because I'm a Conservative."
Mr Chichester said he backed same-sex civil unions having equal rights to traditional marriage in areas including property and taxation, but he could not agree "being equal must mean being the same".
He went on: "For two obvious reasons, these unions or partnerships are not the same, namely a marriage is about a man and woman joined together for the procreation of children. What you are doing in attempting to redefine marriage is to undermine and affront the traditional majority to no benefit to the gay and lesbian minority."