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David Cameron will risk a major battle with his party next week by backing gay weddings in churches, the Evening Standard can reveal.

He will go further than ever in his modernising drive by saying religious groups should be allowed to host same-sex civil weddings in churches, synagogues and other religious buildings if they choose.

Organisations that reject gay marriage, such as the Church of England and the Roman Catholic Church, will have legal protection from being forced to host ceremonies against their wishes, the Prime Minister will pledge.

Tory MPs will have a free vote on a Bill next year, while Labour MPs will be whipped in favour.

It means all three party leaders now support a historic equality reform that would once have seemed incredible — that homosexual partners can have the same civil marriage rights as heterosexual couples and even get married in a religious setting. But some Conservatives said there would be “outrage” and warned that the Tories would haemorrhage members.

Tory MP Peter Bone said Mr Cameron’s party was split about 50-50 and claimed that several Cabinet ministers would vote against gay marriage.

He added: “Despite the PM’s assurance, the redefinition of marriage — because of the European Convention on Human Rights — will force churches to marry same-sex couples. This will outrage millions of people and hugely damage the Government in electoral terms.”

Culture Secretary Maria Miller will formally launch the gay marriage revolution on Thursday when she issues the Government’s response to a summer-long consultation on the plans. There have been “hundreds of thousands” of responses, say insiders, including a large number of protests by church-goers.

Crucially, government lawyers told Ms Miller they have devised a foolproof legal “lock” to protect churches that oppose the reform from being dragged in. This will be written into legislation and the Government will also go to Europe to seek guarantees.

Nick Clegg was the first heavyweight politician to back the idea, saying in an Evening Standard interview in June: “We shouldn’t stop any church that wants to conduct gay marriage.”

Mr Cameron’s view is that there is no reason to bar other churches from voluntarily hosting same-sex weddings, providing the “locks” ensure no church will be forced to take part.

Quakers and some Liberal Jews have loudly called for the right to marry gay worshippers, saying it is unfair to exclude some members of the congregation from the joys of marriage.

The consultation is thought to have revealed a blanket ban on church weddings would be open to legal challenge. This means churches such as the C of E are better protected if religious organisations that want to conduct same-sex marriage are permitted to do so.

Mr Cameron is prepared for a row with his MPs, say sources, because he believes the public are relaxed about the idea. Several polls have shown clear backing for gay weddings among voters, with only older people and regular church-goers strongly opposed.

A government spokesman said: “We are committed to bringing equal civil marriage forward and the consultation results will be announced next week. We are very clear that religious organisations must be protected and that none will be forced to conduct same-sex marriage.

“EU law is very clear that this is the case and we will additionally bring in very strong legal locks to ensure that this is watertight.”

Equality campaigners welcomed the news. Pressure group Out4Marriage said it was “delighted” the coalition was making the Bill a priority. “We are glad that they appear like us to believe in religious freedom, that churches must have the freedom to decide themselves whether to allow gay couples to marry” it said. “We eagerly await the full details of this historic change in the law next week.”