Culture Secretary Maria Miller has insisted there is "clear support" from within the Conservative Party for gay marriage as David Cameron faces being deserted by more than half of his MPs over the issue.

Mrs Miller highlighted a letter from Mr Cameron's three most senior Cabinet allies - Chancellor George Osborne, Foreign Secretary William Hague and Home Secretary Theresa May - in support of same-sex marriage, saying there was "significant support" from key Tory activists around the country for the legislation.

"I would point out that today not only have we had a letter from the Home Secretary, the Chancellor and the Foreign Secretary in the papers, but also significant support, again, in the letters of the papers from some of our key activists around the country," she told BBC Breakfast.

"I don't think it is quite as cut and dried as you suggest. Yes, there is a difference of opinion and yes, some people have very principled religious beliefs on this issue, but there is clear support within my party and indeed within the other major parties."

Her remarks were made as the Prime Minister will join the large majority of Labour and Liberal Democrat MPs in voting for the legalisation of same-sex marriage in the teeth of opposition from Tory traditionalists ranging from the party's grassroots up to the Cabinet.

Some estimates put the number of Tory MPs against the measure at upwards of 150, including Environment Secretary Owen Paterson among other ministers.

On the eve of the vote, the Chancellor, the Foreign Secretary and Home Secretary sought to win over wavering MPs by insisting gay marriage was "the right thing to do at the right time".

In a joint letter to The Daily Telegraph, they questioned whether it was "any longer acceptable to exclude people from marriage simply because they love someone of the same sex". "Marriage has evolved over time. We believe that opening it up to same-sex couples will strengthen, not weaken, the institution," they wrote.

Labour leader Ed Miliband is expecting in the region of 25 of his own MPs to vote against the plan, although all of the shadow cabinet will support the Marriage (Same Sex Couples) Bill at its crucial Second Reading.

But the proposals - which are opposed by the Church of England and its new Archbishop of Canterbury Justin Welby - should still pass easily as they are backed by the vast majority of Labour and Liberal Democrat MPs. Mr Miliband said he would "proudly" vote in favour and would actively urge his MPs to join him in making "an important step forward in the fight for equality in Britain".