The Government will set out its plans to legislate for gay marriage amid bitter opposition from some Tory MPs.
The historic move - which will allow same-sex couples to wed in church - has been hailed by gay rights campaigners and is seen as a litmus test of David Cameron's efforts to modernise the Conservative Party.
But the move has angered Tory traditionalists who have warned that many activists will no longer be prepared to go out and campaign for the party if it goes through. The Prime Minister on Monday sought to defuse tensions within the party, assuring Conservative MPs that they will have a free vote on the issue.
With both Labour and the Liberal Democrats backing change, the legislation - to be unveiled in the Commons by Culture Secretary Maria Miller - is expected to pass by a comfortable majority. But it is thought that as many as 130 Tory MPs could vote against, underlining the deep divisions in the Conservative ranks.
Mr Cameron on Monday acknowledged that equal marriage was not seen as a priority at a time when the Government was preoccupied with deficit reduction and public sector reform, but said he believed the time had now come for change.
"Civil partnerships were a great step forward, but I think marriage is a great institution, so why close it off to people who happen to be gay? So we should have marriage equality," he told a Westminster lunch.
"There are those in the Conservative Party who disagree. I have always said it is going to be a free vote and that free vote applies to everybody, so I won't be whipping people or pressurising people.
"This is a matter for Parliament. I think we just have to be grown up and accept that in a modern party, sometimes we will have issues of conscience where people will vote in different ways."
The measures will grant religious groups which want to stage same-sex civil marriages in places of worship the right to do so, but will offer a guarantee that no institution will be forced to do so. Mrs Miller has told MPs the plans had been drafted so there was a "negligible" chance of a successful legal challenge to religions which refuse to open their doors to gay weddings.
But senior Conservative backbencher Edward Leigh, who used an urgent question to force the Culture Secretary to come to the despatch box, said the state had "no right" to redefine marriage in this way. He called for a new consultation, arguing that the current plan "greatly increases the chance of human rights litigation to force churches, against their will, to have same-sex marriages".