Government proposals to lift the ban on civil partnership ceremonies being held in places of worship will bring it into conflict with thousands of evangelical churches, the Church of England and the Roman Catholic Church, some religious groups have said.
"Christians will need a great deal of reassurance that the Government is not about to do something that will make their situation even worse," they added.
"We reiterate our long-held opposition to allowing civil partnerships to be registered in churches. It is a breach of undertakings made by government ministers during debates on the Civil Partnership Bill.
"Parliament was persuaded to pass that Bill, in part, because it was made clear that civil partnership was a civil rather than a religious institution and would not take place in religious premises."
The groups, which include Affinity, The Christian Institute, Christian Concern, Reform and the Fellowship of Independent Evangelical Churches, went on: "In any legislation, churches must be protected against the possibility, now and in the future, of any kind of legal action being brought against churches which conscientiously disagree with civil partnerships.
"When it comes to equality legislation, permission often turns rapidly into coercion.
"In a country where faith-based adoption agencies have been forced to close or cut their religious ties by equality law, where Christian marriage registrars can be dismissed for their religious views on marriage and where Christian B&B owners are forced to pay compensation to same-sex couples, Christians will need a great deal of reassurance that the Government is not about to do something that will make their situation even worse."
The Church of England has already said that it will not allow any of its churches to be used for civil partnership ceremonies.