David Cameron has said allowing gay marriage will "make our society stronger" ahead of a vote on the issue that will expose deep divisions in the Tory party.
In a televised statement recorded less than two hours before the vote, the Prime Minister said: "Today is an important day. I am a strong believer in marriage. It helps people commit to each other and I think it is right that gay people should be able to get married too.
"This is, yes, about equality. But it is also about making our society stronger.
"I know there are strong views on both side of the argument - I accept that. But I think this is an important step forward for our country."
More than 100 Conservative MPs are expected to defy the Prime Minister and oppose the bitterly controversial Marriage (Same Sex Couples) Bill. The vote will be live on Sky News at 7pm.
Former coalition minister Sir Gerald Howarth said the Government had no mandate for such "massive social and cultural change".
"There are many major issues this country has to deal with. This is an irrelevance," he said.
There is also anger at the decision to whip a vote on the timetable for the Bill, with many backbenchers believing it should be scrutinised by the House rather than a committee of MPs.
Earlier, Culture Secretary Maria Miller told MPs at the start of a Commons debate that the Government's plans would protect freedom of faith and extend equality to gay couples.
"Suggestions that this Bill changes something that has remained unchanged for centuries simply does not recognise the road that marriage has travelled as an institution," she said.
She also played down claims that churches refusing to hold gay weddings could face legal action, calling the prospect of a challenge from the European Court of Human Rights "simply inconceivable".
"This Bill is about one thing. It is about fairness, it is about giving those who want to get married the opportunity to do so whilst protecting the rights of those who don't agree with same-sex marriage," she said.
William Hague, George Osborne and Theresa May also joined forces in a last-minute appeal, writing an open letter insisting it "is the right thing do do at the right time".
The three most senior Tories in the Cabinet questioned whether it was "any longer acceptable to exclude people from marriage simply because they love someone of the same sex".
The legislation will get a second reading with Labour and Liberal Democrat support but it is possible that more than half the Tories' 305 MPs could vote against it.
This would be hugely damaging for the Prime Minister, coming amid a febrile atmosphere of plotting against him and claims that hundreds of Tory activists are deserting the party in protest.
New poll results also show that that the issue threatens to drive voters away from the Tories.
A YouGov survey for The Sun put Labour ahead by 15 points with the Tories on 30%.
In a separate ComRes poll for ITV News, more than a third - 34% - said the move made the Conservatives less attractive to them as voters. Only 15% said it made the party more appealing.
All three major parties have allowed a free vote on the Bill. Around 20 Labour MPs, a few Lib Dems and the Democratic Unionist Party's MPs are also expected to vote against.
Lib Dem MP John Pugh said he would break ranks with his party because there was a "good liberal case" against the move.
In an open letter to constituents, he said his fundamental objection was that the legislation "achieves none of its objectives and weakens the link between marriage and the family".
But the Tories are potentially even more divided on gay marriage than they are on Europe, with party activists as well as Conservative MPs publicly clashing over the proposals to allow same sex couples to marry.
At least two members of the Cabinet, Environment Secretary Owen Paterson and Welsh Secretary David Jones, are expected to oppose the proposals.
Two more, Defence Secretary Philip Hammond and Work and Pensions Secretary Iain Duncan Smith, may abstain.
In a bizarre twist ahead of the vote, gay political commentator Iain Dale claimed that the Tories threatening to vote against the Bill include several "closet-case gays" including "two supposedly heterosexual MPs who I know to be conducting gay affairs".