British Prime Minister David Cameron says he backs gay marriage because he's a “massive” supporter of marriage.

As lawmakers prepare to vote on a measure legalizing marriage equality, the government is facing harsh criticism after it reversed course on the controversial issue of allowing gay and lesbian couples to get married in a church and other religious premises.

Some religious groups, including Quakers, Unitarians and Liberal Judaism, called for the change, while the Church of England and the Roman Catholic Church have strongly condemned the marriage reform.

“For Quakers, this is an issue of religious freedom and we don't seek to impose this on others,” Paul Parker, recording clerk for Quakers in Britain, is quoted as saying by the BBC.

Colin Hart, campaign director for the Coalition For Marriage, which opposes marriage equality, called the proposal “alarming.”

“The suggestion that by creating an 'opt-in system' you somehow prevent churches, mosques and synagogues being sued is risible,” he said. “This is now made much more likely.”

Cameron defended his proposals.

“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 percent 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,” he added.