Ed Miliband has called for churches to be able to hold same-sex marriages if they want to "provide that opportunity", as part of a campaign for equal marriage.
In a video for the Out4Marriage campaign the Labour Party leader said: "I think, whether you are gay or straight, you should be able to signify your commitment, your love with the term marriage.
"We will be pushing the government to get on with the process for legislating for equal marriage, and we'll also be saying to them, where faith groups want to provide that opportunity for gay couples as well as straight couples, they should be able to do so.
"We've come a long way on gay and lesbian rights in the last two decades - civil partnerships, age of consent, a range of other things - but I know there's further to go.
"I know that equal marriage is a very, very important part of ensuring equality before the law, the equality in our culture which is so important, and such a sign of us being a modern country and the kind of country I believe in."
The current proposals for equal marriage in England and Wales do not include religious marriages, but the Scottish government has said it intends to legislate for both civil and religious marriages for same-sex couples.
Benjamin Cohen, of Out4Marriage, told The Huffington Post UK: "This is a key part of our campaign. Our first ever video recorded for the campaign was Yvette Cooper who backed religious marriage but did so in a personal capacity.
"David Cameron and Nick Clegg backed religious marriage, but made it clear they were doing it in a personal capacity too. Ed Miliband has done it on behalf of the party, which is a big step."
He added: "We have made it clear that if there is no amendment allowing religious marriage then there will be a legal challenge, on the grounds of religious freedom, because the government will be preventing churches and synagogues from making their own decisions on the matter."
The Church of England has opposed plans to allow marriage for gay couples, claiming it would "alter the intrinsic nature of marriage as the union of a man and a woman, as enshrined in human institutions throughout history" and a spokesman for the Catholic Church in Scotland sparked fury when he described the plans as a "dangerous experiment."
Other faith groups have also opposed plans for same-sex marriage. The Muslim Council of Britain has launched a website, Muslims Defending Marriage, which describes the civil partnerships system as a "ploy" designed to "attack traditional marriage".
The Chief Rabbi and the London Beth Din, representing the largest strand of Orthodox Judaism in the UK, have also opposed plans for same-sex marriage.
But representatives from Quakers, the Movement for Reform Judaism, Liberal Judaism and the Unitarian and Free Christian Churches have all pledged to support equal marriage and wish to be able to hold gay marriages in their churches and synagogues.
Michael Bartlet, parliamentary liaison secretary for Quakers in Britain, told The Huffington Post UK: "Marriage should be a commitment available for all heterosexual and gay couples, and religious organisations who want to marry gay couples should be allowed to do so.
"We are pleased at any step towards this, across the political spectrum, because we think this is something that needs cross-party support."
Ben Rich, chief executive of the Movement for Reform Judaism, told The Huffington Post UK: "We think that there should be marriage equality, we want the rules on marriage to be exactly the same for everyone.
"We do welcome this step by the Labour party, and the more this has cross party support, the better.
"However, it is right that individuals must be able to choose if they want to conduct same sex marriages, and if they don't want to, that's fine. As long as they don't try and prevent that couple getting married somewhere else."
But Andrea Minichiello Williams, chief executive of Christian Concern, said:“Mr Miliband’s comments contain well-worn phrases which hide the reality of the effects that redefining marriage will have.
“When it comes to allowing churches to perform same-sex ‘marriages’, permission will become coercion. Those churches which continue to practice biblical teaching on marriage will feel immense pressure to conform to the State’s new definition."