Liberal Democrat MPs are being encouraged to vote in favour of a Manuscript Amendment to the Marriage (same sex couples) Bill, which would start an immediate consultation on civil partnerships for straight couples.

The Labour party has put forward the Manuscript Amendment, which aims to start an immediate consultation on civil partnerships for straight couples.

It can only be considered if Culture Secretary and Minister for Equalities, Maria Miller’s amendment for a future review on civil partnerships is approved.

From a Lib Dem source, the Guardian published a message circulated urging MPs to vote in favour of the manuscript amendment but against a previously tabled amendment by three Tory MPs – Tim Loughton, Charlotte Leslie and Rob Wilson – which pushes for civil partnerships to be an option for heterosexual couples as part of the bill.

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

The message from the Liberal Democrats is available to read below.

As Nick Clegg said earlier today, the overriding objective of the Liberal Democrats is to ensure that equal marriage becomes law. While tonight’s votes are free votes, Liberal Democrat MPs will be strongly encouraged to vote in the way that ensures the progress of Marriage (Same-Sex Couples) Bill is not jeopardised. That means our MPs will be strongly encouraged to support Labour’s amendment to the proposed consultation and then to oppose Tim Loughton’s amendment on mixed-sex couples civil partnerships. The Liberal Democrats support the extension of civil partnerships to mixed sex couples, but not at the expense of the progress of equal marriage.

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

Openly gay Tory MP, Nick Herbert, has also circulated an email urging Conservative MPs to vote in favour of the Manuscript Amendment, but against the Loughton amendment.

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.

But on Monday, Maria Miller stated that the eventual date for this would be “next summer”.