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.
Cameron has championed the drive to allow same-sex couples to marry, but it is 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 that 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 Cameron's Downing Street office urging him to delay any parliamentary decision on gay marriage until after the next election.
The letter warned there would be "significant damage to the Conservative Party in the run-up to the 2015 election" if the plans went ahead.
A ComRes poll of just over 2,000 people who voted Conservative at the last general election found that 20 percent 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 published in the Daily Telegraph newspaper on Saturday also found that 62 percent of voters overall thought Cameron's chief motivation was trying to make the Conservatives seem "trendy and modern".
But 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 said giving same-sex couples the right to marry was "the right thing to do now and for the future".
Miller also insisted the new legislation could not be challenged in the European Court of Human Rights.
The Conservatives' junior coalition partners, the Liberal Democrats, and the opposition Labour party are broadly in favour of the gay marriage legislation.
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.
This provision was seen as a way to allay fears that future legislation could require the churches to do so.
Other religious institutions can "opt in" if they wish to conduct ceremonies.
The new Archbishop of Canterbury Justin Welby, the spiritual leader of the world's Anglicans, said he stood by the Church of England's criticism of the legislation, as he was formally confirmed in office.
He said: "I have no idea how the vote will go, so I am not going to get into hypothetical questions.
"I stand, as I have always stood over the last few months, with the statement I made at the announcement of my appointment, which is that I support the Church of England's position on this."
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.