Straight couples could be allowed to enter civil partnerships, rather than get married, under proposals to be voted on by MPs.
MPs have tabled new amendments to gay marriage legislation currently going through Parliament to give straight couples the same rights as homosexual ones.
Campaigners said the amendment showed the changes risked weakening marriage by allowing straight couples to enter civil partnerships.
The measure, which is due to be voted on by MPs in the Commons at the end of May, would allow couples to be in civil partnerships rather than be married.
Tim Loughton, former Children’s minister, tabled the amendment to Marriage (Same Sex Couples) Bill in the Commons on Tuesday.
He said it was likely there would be “widespread support” for it when the amendment was voted on by MPs on the floor of the Commons next month.
Colin Hart, director of the Coalition for Marriage, said: “This is yet another amendment that pushes the redefinition of marriage beyond the consultation the Government has shifted its position constantly and this will report a further weakening of the institution of marriage.
The amendment will be seen as a direct challenge to Prime Minister David Cameron who suggested last month that he was against extending civil partnership rights to straight couples after MPs voted overwhelmingly in favour of gay marriage.
Asked by Christopher Chope MP if he will “ensure that civil partnerships are open to heterosexual couples on an equal basis with homosexual couples”, he replied: “I will obviously listen carefully to what he says.
“But frankly I am a marriage man, I am a great supporter of marriage. I want to promote marriage, defend marriage, encourage marriage.
“The great thing about last night’s vote is that two gay people who love each other will now be able to get married. That is an important advance. I think we should be promoting marriage rather than looking at any other way of weakening it.”