LONDON (AFP) - British Prime Minister David Cameron faces a battle to placate traditionalists in his Conservative Party bitterly opposed to gay marriage ahead of a parliamentary vote on the contentious issue on Tuesday.

Mr Cameron has given strong backing to the plans to allow same-sex couples to marry, but they are fast becoming a highly divisive issue for his party.

The prime minister has promised lawmakers a free vote on the proposed legislation in the House of Commons on Tuesday, meaning party managers will not try to influence their choice.

But there is concern in Conservative ranks that more than half of the party's MPs could vote against the bill.

Many grassroots Tories are warning that a green light for gay marriage could alienate rank-and-file Conservatives.

On Sunday, more than 20 current and former constituency chairmen delivered a letter to Mr Cameron's Downing Street office urging the prime minister to delay any parliamentary decision on gay marriage until after the next election.

The letter warned that if the plans pass into law, there would be "significant damage to the Conservative Party in the run-up to the 2015 election".

A ComRes poll of just over 2,000 people who voted Conservative at the 2010 general election found that 20 per cent agreed that they "would have considered voting Conservative at the next election but will definitely not if the coalition government legalises same-sex marriage".

The survey in the Daily Telegraph newspaper also found that 62 per cent of voters overall thought Mr Cameron's chief motivation was trying to make the Conservatives seem "trendy and modern".

But Ms Maria Miller, the Conservative culture and equalities minister, warned opponents of gay marriage that they were out of step with social progress.

In an article in The Times on Monday, she dismisses the concerns of traditionalists who believe marriage should be the preserve of heterosexuals, arguing that giving same-sex couples the right to marry was "the right thing to do now and for the future".

Ms Miller also insisted the new legislation could not be challenged in the European Court of Human Rights.

Former children's minister Tim Loughton said: "This is a wake-up call to just how damaging an issue gay marriage is for the Conservative Party.

"Many stalwart Conservative supporters are feeling pretty bruised by this issue which came out of nowhere, didn't feature in the manifesto and is now being forced through by the government that seems to want to pick a fight with its own supporters."

The Conservatives' junior coalition partners, the Liberal Democrats, and the opposition Labour party are broadly in favour of the gay marriage legislation.

But the new Archbishop of Canterbury Justin Welby, the spiritual leader of the world's Anglicans, is expected to reveal after his official confirmation on Monday that he believes marriage should be between a man and a woman.

In plans unveiled in December, the government said it was proposing to allow same-sex couples to marry, but would explicitly ban the established Churches of England and Wales - which are opposed to the legislation - from conducting ceremonies.

Other religious institutions can "opt in" if they wish to conduct ceremonies.

Gay couples in Britain have had the right to enter into a civil partnership since 2005.

Civil partnerships offer identical rights and responsibilities to civil marriage, although campaigners point to some differences such as international recognition which applies to marriage but not partnerships.

The vote comes after France's National Assembly on Saturday overwhelmingly approved a key piece of legislation that will allow homosexual couples to marry and adopt children, to the delight of gay activists.