Update 13:25: Downing Street originally told PinkNews that no option on the equal marriage bill would be ruled out until after the vote on the Tim Loughton amendment, however another spokesman has confirmed that David Cameron is in full support of the bill, but will decide the best way to make it happen, after the vote.

The Government may withdraw its support for the pending equal marriage bill for England and Wales, if an amendment to allow civil partnerships to straight couples passes.

A Downing Street source told PinkNews.co.uk that, if an amendment put forward by Tim Loughton, which would extend civil partnerships to heterosexual couples, were to pass, the Government may pull its support for the Marriage (Same Sex couples) Bill.

The source said no option could be ruled out over the bill, until after the vote on the amendment.

Three Tory MPs – Tim Loughton, Charlotte Leslie and Rob Wilson – are pushing for civil partnerships to be an option for heterosexual couples in an amendment as part of this week’s third reading of the Marriage (Same Sex Couples) Bill.

Prime Minister David Cameron has said that he holds the same position as Culture Secretary and Minister for Equalities, Maria Miller, who warned that the proposed backbench amendment on civil partnerships for heterosexuals could dramatically delay implementation of the same-sex marriage bill.

The Prime Minister has made it clear that he is to vote against the amendment, Downing Street has said.

“I would expect the Prime Minister to be voting tonight. He will vote in the same way as the Secretary of State for culture,” David Cameron’s spokesman said.

At the lobby briefing, the spokesman said that Mr Cameron’s emphasis was on allowing same-sex couples to marry.

“The Prime Minister’s view is very much Maria Miller’s, which is that the bill is about extending marriage to same-sex couples.

“The Equal Marriage Bill is all about the extension of marriage. There have been concerns and issues raised in the House with regard to civil partnerships and that’s why the Government has proposed to formally review civil partnerships.”

However, on the bill, Downing Street said the Government “has a legislative programme and it is getting on with it”, which has been taken as a signal that the Government will not drop the bill.

Some have said that Downing Street’s position on the amendment may be a precautionary measure to ensure that Labour MPs do not support the amendment. Many Labour MPs, however, are expected to support it.

However, some in Westminster fear the amendment is an attempt to “wreck the bill” because it could delay its passage beyond the 2015 general election.

Liberal Democrat leader and Deputy Prime Minister Nick Clegg has said he will not allow Tory opponents to succeed in wrecking the Marriage (Same Sex Couples) Bill.