David Cameron wants churches in England and Wales to be allowed to conduct same-sex marriage ceremonies.
He said he did not want gay people to be "excluded from a great institution", but would not force any groups to hold ceremonies in their places of worship.
Ministers will reveal their response to a consultation next week. MPs will be given a free vote on the issue.
The Church of England said it would study the proposals but was firmly against same-sex marriage.
In a statement, the Church said: "We believe that redefining marriage to include same-sex relationships will entail a dilution in the meaning of marriage for everyone by excluding the fundamental complementarity of men and women from the social and legal definition of marriage.
"Given the absence of any manifesto commitment for these proposals - and the absence of any commitment in the most recent Queen's Speech - there will need to be an overwhelming mandate from the consultation to move forward with these proposals and make them a legislative priority."
The Church said its stance was not a "knee-jerk resistance to change", but was "motivated by a concern for the good of all in society".
Mr Cameron's proposals have also angered some Tory MPs who have opposed the change in the law.
Bob Blackman, MP for Harrow East, warned there will be "outrage throughout the country" and a "clear backlash" against David Cameron's proposals.
He told BBC News: "Marriage is between one man and one woman and so changes to the definition of marriage are not appreciated and not strongly supported."
He said he was against teachers "being forced to say same sex relationships are the equivalent of heterosexual relationships" - something he said would happen if the move become law.
Labour and the Lib Dems have yet to decide whether to join the Conservatives in granting their MPs a free vote on the issue, although the majority of MPs in the three parties are thought likely to back it.
But Mr Blackman predicted the legislation could face "an interesting challenge" in the House of Lords.
Conservative MP Stewart Jackson tweeted that the Bill would be "massacred in the Lords", adding, in a swipe at the prime minister: "Arrogant Cameron knows best."
Mr Cameron said: "I'm a massive supporter of marriage and I don't want gay people to be excluded from a great institution.
"But let me be absolutely 100% clear: if there is any church or any synagogue or any mosque that doesn't want to have a gay marriage it will not, absolutely must not, be forced to hold it.
"That is absolutely clear in the legislation.
"Also let me make clear, this is a free vote for Members of Parliament, but personally I will be supporting it."
The government's initial consultation document said it would not be possible for a same-sex couple to get married in church and other religious premises.
Under the new proposals, due to be outlined next week by Equalities Minister Maria Miller, religious organisations which do not want to host same-sex weddings will be given an absolute guarantee they will not be forced to do so.
But Whitehall sources say the best way to make the guarantee "water-tight" is to allow religions to opt in to hosting same-sex ceremonies if they want to.
Labour leader Ed Miliband and deputy prime minister Nick Clegg also support gay marriage in church.
Mr Clegg said: "It's very important to remember that in our plans we're not going to force any church or any religious denomination to hold same-sex marriage ceremonies if they don't want to but I do think it's time that we allow any couple, no matter who they are, to marry if that's what they want to do."
Shadow Home Secretary Yvette Cooper said: "I hope David Cameron will not be deterred by opposition within his own party and beyond.
"We need the government to move forward with an early debate in Parliament so the issue doesn't stall."
The Church of England and Roman Catholics, among other denominations, have voiced opposition to same-sex marriage.
But some religious groups, including Quakers, Unitarians and Liberal Judaism, are in favour of gay marriage and are thought likely to apply to be allowed to stage ceremonies.
Paul Parker, recording clerk for Quakers in Britain, said: "For Quakers, this is an issue of religious freedom and we don't seek to impose this on others."
Benjamin Cohen, of Out4Marriage, which backs same-sex weddings, said he was "delighted" by Mr Cameron's announcement.
"Legislation must give individual religious organisations the freedom to decide for themselves whether to hold same-sex marriages.
"None should be forced to, but those that wish to must be given the rights to do so."
Colin Hart, campaign director for the Coalition For Marriage, which opposes same-sex marriage, said: "The decision to press ahead with the profoundly undemocratic proposals to rewrite the traditional meaning of marriage is deeply disappointing and regrettable.
"What is even more alarming is the PM has gone back on his promises that churches will be protected.
"The suggestion that by creating an 'opt-in system' you somehow prevent churches, mosques and synagogues being sued is risible. This is now made much more likely."