The London Mayor and Education Secretary Michael Gove are two of 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 a Commons vote next year.
Some have criticised David Cameron for "arrogance" in pushing ahead with the plans, despite his promise of a free vote.
Under the proposals, due to be fleshed out this week, churches and other venues will be allowed to "opt in" to holding civil marriage ceremonies.
Ministers will offer a guarantee that no institution will be forced to marry gay couples - but religious groups fear this won't stand up to challenges under the European Human Rights Act.