Writing exclusively for PinkNews.co.uk, Liberal Democrat MP Stephen Gilbert says today’s start of the third reading of the Marriage (Same Sex Couples) Bill is a huge test for David Cameron’s commitment to marriage equality and for the progressive elements within the Conservative Party.

May has been a good month for equal marriage. Minnesota became the twelfth American state, plus Washington DC, to allow same-sex couples to marry and France became the ninth country in Europe (the fourteenth around the world) to introduce this landmark change. Millions of gay and lesbian people are now able to marry the person they love.

Today, in Britain, the debate returns to the House of Commons. I have no doubt that a significant majority of MPs are in favour of equal marriage. But let’s also be in no doubt that a small number of out of touch Tory MPs are using any and all means at their disposal to disrupt and derail this historic legislation. How David Cameron deals with these detractors will be a litmus test for his leadership as he tries to re-establish control over his fractious party.

I don’t doubt the prime minister’s personal commitment to delivering equal marriage. My concern is that David Cameron is in danger of being buffeted by the storms raging within his disunited party. Let’s be clear, Cameron’s party is at war – the Compassionate Conservatives versus the Toxic Tories. They seem unable to agree a narrative in response to the threat, as they see it, from the rise of UKIP and the “swivel-eyed-loons” inside and outside the ranks of Conservative MPs are pushing for an ever more right-wing agenda – like the ditching of equal marriage and an immediate referendum on membership of the EU.

The Conservative Party of 2013 is not the same party that entered the Coalition Government in May 2010. David Cameron promised the British people a centrist approach – a compassionate Conservative Party that was outward looking, internationalist and environmentally savvy. Three years later we have a Tory party that reverted to type, turned inward and speaks to its own shibboleths. Today’s Tories seem ungovernable and reluctant to govern.

It would be a tragedy if, as the rest of the world makes progress on equal marriage, the internal machinations of a Conservative Party at war cause the equal marriage bill to be derailed in Britain. To avoid this outcome the prime minister may have to accept that there are likely to be majorities in the House of Commons for opposite sex civil partnerships, humanist weddings as well as equal marriage for gay and lesbian people. He should welcome these changes.

David Cameron now faces a fundamental choice. He can continue to allow his party to lurch to the right and further diminish his chances of spending another five years in Number 10 or he can deliver the Conservative Party that he promised the British people in 2010.

What the prime minister does over the next three days will make a huge difference to the millions of gay and lesbian people in Britain, but it will also tell us a lot about the direction of his party.

Stephen Gilbert is the Liberal Democrat MP for St Austell & Newquay and was the first MP to propose equal marriage at the Liberal Democrat Party Conference in September 2010.