A Tory former minister is calling on the Government to close an "inequality loophole" and allow opposite sex couples to have a civil partnership amid fears the proposal could be blocked.
Tim Loughton believes the change should be made on equality grounds, to remedy the "ignorance" of unmarried couples who believe they are protected in common law and to boost family stability.
But the MP believes the Government will block the move which is set out in his Civil Partnership Act 2004 (Amendment) Bill.
The Bill, which would amend the Act so that opposite sex couples can enter into a civil partnership, is currently scheduled to be the second draft law to be discussed in the Commons on Friday.
Mr Loughton expects the Government to "talk out" the legislation to stop it from progressing.
He told the Press Association: "We will get some time to debate it. I have been told that the number one Bill will stop earlier on and then we can debate it but then the minister will talk it out."
Mr Loughton said he believed the Government "just doesn't see it as a priority and it's too much of a hassle to sort of open it all up again".
He suggested it is unfair only same sex couples are able to enter into a civil partnership while both same sex and opposite sex couples are able to get married as he called on ministers to address the situation.
He said: "If they treated the same sex marriage bill as an equality issue then they should be equally concerned to close an inequality loophole which that Bill created."
Mr Loughton said same sex marriage, brought in by the Marriage (Same Sex Couples) Act 2013, had "inadvertently created a new inequality in that civil partnerships are only available to same sex couples" as he argued why the law should be changed.
He continued: "Secondly, there is a potential danger of ignorance of many people at the moment that there is such a thing as common law wife or common law husband and given that just under three million couples are opposite sex cohabiting couples who are not married they are not subject to any of the protections.
"If one of them were to die suddenly, inheritance tax goes out of the window, they don't have any legal status in law, protections or anything like that.
"There is a sort of general ignorance that that status is somehow recognised in law. It's not."
He added: "Thirdly it's a family stability issue that if you give some way of recognising that relationship in the eyes of the state, because it is not recognised by the state, it is likely to lead to a more stable relationship and that must be good for the over half of children who are now born into cohabiting couples rather than married couples."
Mr Loughton said the change should be made because many couples who do not want to get married "want to have something which gives them status in law and also then shows publicly a commitment to each other".
The MP said he intends to try to amend future suitable legislation if his Bill fails to progress on Friday.
"It's not going to go away," he said.