Prime Minister David Cameron will reportedly speak out in favour of equal marriage later this evening, ahead of tomorrow’s first vote by MPs on the Marriage (Same Sex Couples) Bill, according to the Times.

Twenty current and former constituency chairmen have warned Mr Cameron that legalising equal marriage could harm Tory election prospects. 

Reports suggest upwards of 120 Tory MPs could vote against the bill, including some cabinet ministers.

Michael Fabricant, a Tory vice-chairman, said he was “disturbed” to hear of ministerial aides warning backbenchers that their careers would be dented if they failed to support the government – even though Mr Cameron has given his MPs a free vote.

Yesterday, Foreign Secretary William Hague said the Conservative Party would look out of touch if they failed to support marriage equality.

On Monday, in an article in the Times newspaper, Culture Secretary Maria Miller said being gay was not a good reason to prevent couples marrying.

Recognising the split in the Conservative Party over the issue, Mrs Miller conceded: “As a party we have grappled with many decisions that have not been plain sailing and we have not always found easy accord, whether that is within Westminster or within our current coalition.”

She also said the bill would be safe from challenge via the European Court of Human Rights.

It will receive its second reading tomorrow by an overwhelming majority, thanks to support from Labour and Liberal Democrat MPs.

However, Downing Street is anxious that Mr Cameron and Tory supporters of equal marriage are not outnumbered by opponents.

Up to 130 of the party’s 303 MPs are expected to support the bill, with a similar number having indicated their opposition.