The depth of Conservative division over gay marriage is to be laid bare when David Cameron faces being deserted by more than half of his MPs over the highly-charged issue.

In the teeth of intense opposition from Tory traditionalists from the party's grassroots all the way up to the Cabinet, the Prime Minister will join the large majority of Labour and Liberal Democrat MPs in voting for the legalisation of same-sex marriage.

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, Mr Cameron's three most senior Cabinet allies - Chancellor George Osborne, Foreign Secretary William Hague and Home Secretary Theresa May - 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. "Attitudes towards gay people have changed. A substantial majority of the public now favour allowing same sex couples to marry, and support has increased rapidly. This is the right thing to do at the right time."

Justice Secretary Chris Grayling also spoke out in favour - telling gay magazine Attitude it was a "sensible next step" in tackling homophobia in the UK.

All three main party leaders are allowing a free vote on what is seen as a conscience issue, although the programme motion for the timetabling of the Bill's passage through parliament is whipped and could serve as a test of the Prime Minister's authority.

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.