* British PM Cameron's Conservatives split over legalising gay marriage
* Lower house of parliament to vote on issue on Tuesday
* Law likely to get initial approval in face of rebellion
* Revolt comes amid talk of leadership challenge against PM
British Prime Minister David Cameron is expected to see his ruling Conservative party split in two on Tuesday over his government's plans to legalise gay marriage, a move critics say is not a priority for the public and unnecessarily divisive.
Although parliament is likely to vote to give the draft law initial approval, up to half of Cameron's 303 Conservative lawmakers, including some Cabinet ministers, are expected to vote against it on what they say are moral and religious grounds.
In a sign of how divisive the issue has become for Cameron, the finance minister, the foreign secretary and the interior minister wrote a joint letter to a national newspaper on Tuesday, urging fellow Conservatives to vote in favour.
Behind in the polls, Cameron is trying to perform a tricky, and some analysts believe, impossible balancing act: to reconcile his desire to show his party is progressive with the views of many of those inside it uncomfortable with such reform.
Amid growing talk of a possible leadership challenge against him, many Conservative lawmakers say they feel Cameron is not a real conservative and is sacrificing what were once core party values on the altar of populism.
"He hasn't got a lot of political capital left in the bank," Stewart Jackson, a Conservative MP who opposes the gay marriage bill, told Reuters. "There is only so much the Conservative party is going to take. He has to deliver some authentic Conservative policies very soon."
Such talk is rife among some Conservative lawmakers and follows a spate of articles in the British press in which a handful of MPs raised the possibility of replacing Cameron with someone else, a prospect most commentators regard as far-fetched before the next election in 2015.
GRIEVANCES AGAINST CAMERON
Their grievances are numerous: that Cameron is allegedly "arrogant", that he is too fond of the European Union for their liking, that the party's policies are diluted by its coalition partner because Cameron failed to win the last election outright, and a nagging fear that he will not win the next one.
Polls show public opinion is on Cameron's side this time - a YouGov poll for the Sunday Times on Sunday showed 55 percent favoured legalising gay marriage, while 36 percent opposed it.
However, the same poll showed the issue was not one that concerned most voters, placing only 12th in a list of their top 15 priorities ahead of the next election.
The new law proposes legalising same-sex marriage in England and Wales in 2014. It would also allow civil partners to convert their partnerships into marriages.
Gay marriage supporters say while existing civil partnerships for same-sex couples afford the same legal rights as marriage, the distinction implies they are inferior.
Cameron himself said two months ago: "I'm a massive supporter of marriage and I don't want gay people to be excluded from a great institution."
Faced with strong opposition from the Anglican and Catholic churches, the law would not force them to conduct gay marriages, but critics say gay people may launch legal challenges.
Tuesday's vote in the lower house of parliament, the House of Commons, will be "free", meaning MPs from across the political spectrum will be able to vote according to their conscience rather than under party orders.
WARNINGS OVER DIVISIONS
Supported by Cameron's junior coalition partner, the Liberal Democrats, and by the opposition Labour party, the legislation is months and several stages away from becoming law even, if as expected, it is approved on Tuesday.
Ahead of the vote, Conservative party workers urged Cameron to delay the vote, warning the issue could weaken the party and harm his chances of re-election.
"It's divisive in the sense that we didn't campaign in the (2010) election on this issue," David Burrowes, another Conservative lawmaker, told Reuters.
"There are so many other issues we should be concerned about at the moment," he said, adding that dealing with the anaemic economy and overhauling the welfare system should take priority.
Iain Dale, a prominent gay radio presenter and conservative blogger, said Cameron was right to ignore what he called a vocal section of the Conservative party. "Sometimes political leaders have to lead, rather than follow," he wrote on his blog.
In a telephone interview, Dale said he thought "a massive majority" would vote in favour of the new law, saying he didn't think the rebellion would damage Cameron in the long-term.
Others think the fallout could be more corrosive.
Peter Kellner, president of pollster YouGov, said he felt the rebellion would hurt the Conservative brand and its electoral chances with it.
"For Cameron, gay marriage is part of his attempt to persuade the voters that his party belongs to modern, 21st century Britain," he wrote on the pollster's site.
"But the divisions that the gay marriages bill has unleashed ... threaten to send an altogether different message: that the Tories are divided, out of touch and prone to quarrel over issues of little concern to most voters."
With the next election still two-and-a-half years distant, there is also a risk that internal party splits over issues like gay marriage could turn what for now is only talk of a possible leadership challenge into the real thing.
"This comes at a difficult time for the prime minister," said Jackson, the Conservative MP. "There are a lot of people who find him aloof and haughty and don't like his social liberal views. People in the parliamentary party are wondering why he has got us into this position."