Senior Tories including Boris Johnson and Michael Gove have defied party traditionalists by launching a group to campaign for gay marriage.
The London mayor and Education Secretary are among more than a dozen senior figures backing moves to allow same-sex weddings to be held in places of worship.
The issue has sparked a serious rift in the Conservative Party, with more than 100 of its MPs expected to oppose the change in the Commons next year.
Some have criticised David Cameron for "arrogance" in pushing ahead with the plans, despite his promise of a free vote.
Same-sex marriage will remain illegal in the Church of England, as the Government unveiled its plans to extend marriage to gay couples.
David Cameron is facing a backlash from Tory MPs and religious groups over allowing gay weddings in churches.