Legislation allowing gay people to hold civil partnerships in places of worship could be derailed by a group of Tory MPs and peers.
Senior Liberal Democrats have described the threats as showing "reactionary" elements in the Tory party still remain.
Despite the Conservative Party's extensive modernisation project, and David Cameron stating at Tory conference that he supports gay marriage "because I am a Conservative," nine MPs have signed an early day motion calling to repeal changes that allow gay people to get a civil partnership in places of worship.
The move comes as a Conservative peer, Baroness O'Cathain, secured a debate in the Lords calling to scrap the legislation.
Although churches, temples, synagogues and mosques are allowed to refuse to hold same-sex ceremonies, one Tory MP who signed the EDM said it was about making sure churches didn't feel pressured.
Julian Brazier, the MP for Canterbury, maintained it was not about homophobia, saying he knows "lots" of gay people.
And Tory MP Stewart Jackson said the move was to ensure parliament could "have the chance to debate” the changes.
“I support civil partnerships but I think that [there is beginning to be] a predominance of rights of people's over others.
“The government‘s equality office is completely out of step on this issue”.
He added: “It’s not about attacking a particular group, it’s about defending the religious tenets of people in this country.”
But Liberal Democrat Tom Brake warned that the move showed there were "still some reactionary elements" in the Tory party.
He said any move to row-back on allowing gay people to hold civil ceremonies in places of worship would be a "retrograde step" - and emphasised that the views were "not in the mainstream of this coalition government".
Labour sources said the move indicated the Conservative Party had not completed its modernisation project.
"It is typical that some in the Conservative party cannot see the absolute idiocy of opposing a measure that allows those religious faiths and venues who want to to conduct same-sex civil ceremonies on their premises to do so whilst not compelling those that don't."
Edward Leigh, the MP heading up resistance to the measure, told the Catholic Herald the change in law risked causing the same problems which stopped some Catholic agencies from adopting.
“We’ve seen all this before. The Sexual Orientation Regulations went through Parliament without proper scrutiny and they closed down our adoptions agencies as a result. If the Government cares anything about the churches, it will withdraw these regulations and think again.”
But some religious groups support the move. Paul Parker, recording clerk for Quakers in Britain said: “Quakers are keen to hold civil partnerships in our meeting houses and we hope to work with these regulations".
And Rabbi Sandra Kviat, from the organisation Liberal Judaism, added: "We would not want to go back on that at all. Most synagogues and rabbis have no problem with it. It would be a shame if we went backwards in time. It's up to the individual synagogue."