Churches and other religious institutions should be free to stage gay weddings, Ed Miliband said when backing Government plans to legislate for same-sex marriages as a sign that Britain is a "modern country".
However, he went further than ministers in calling for the right to hold same-sex marriages to be extended to faith groups as well.
In a video recorded for the Out4Marriage campaign, he said: "I think, whether you are gay or straight, you should be able to signify your commitment, your love with the term 'marriage', and so the Labour Party provides its wholehearted support for this campaign.
"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 Government is currently consulting on proposals for civil marriages for same-sex couples in England and Wales, with legislation promised before the next general election in 2015.
The Scottish Government, meanwhile, has said it intends to legislate for both civil and religious marriages for same-sex couples in Scotland.
Benjamin Cohen, of Out4Marriage, welcomed Mr Miliband's support for extending the legislation in England and Wales to cover religious ceremonies as well.
"We need full marriage equality, not a half-step that continues to deny gay couples to right to marry in churches, synagogues and meeting houses that are happy to accommodate them. We hope and trust that the Government will come to the same conclusion as the Labour Party," he said.