Culture Secretary and Minister for Equalities, Maria Miller, has warned that a proposed backbench amendment on civil partnerships for heterosexuals could dramatically delay implementation of the same-sex marriage bill.

The Conservative MP said on Monday morning the amendment was a “complicated distraction”.

“I want to see marriages being undertaken under this bill as early as next summer,” she told BBC Radio 4’s Today.

“To put in at this stage such a fundamental change, I believe, risks that and risks significant delay. I think those supporting that need to be aware of that.”

She added: “We want to get this bill focused on what it set out to do which is to open up marriage to more people and make sure it stays absolutely pivotal in society. This is a distraction.”

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.

But in February’s second reading Mr Loughton, a former children’s minister voted against marriage rights for gay couples, and Ms Leslie and Mr Wilson abstained.

Labour is set to support Mr Loughton’s amendment.

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.

Yesterday, Liberal Democrat MP and International Development Minister Lynne Featherstone warned that the amendment may be a “cynical trap”.

The bill will be debated over two days, with its third reading – the final hurdle in the Commons – on Tuesday. If approved, it will go to the House of Lords on Wednesday, where it is expected to face further opposition.

Liberal Democrat leader and Deputy Prime Minister Nick Clegg told PinkNews last December that he was hopeful the first same-sex marriages would be able to take place by the summer of 2013.

The Coalition Government has already set a deadline of the end of the current parliament to introduce equal marriage in England and Wales.