Gay couples will be able to get married in churches under radical government reforms that risk divisions with the Tory right and faith groups.
The Lib Dems believe they have secured a victory for their equalities agenda in allowing same-sex couples to have their civil partnership ceremonies in religious buildings.
Lynne Featherstone, a Lib Dem minister at the Home Office who will make the announcement, also wants greater uniformity between civil partnership and marriage. She has committed the Government to "talking to those with a key interest in the issue of civil partnerships on what the next stage should be". It includes allowing "religious readings, music and symbols", and holds out the possibility of the ceremonies being performed by priests, rabbis or other religious figures.
Last year the Lib Dems adopted a policy allowing civil partnerships to be "converted" into full marriage. Some faiths – including the Quakers, Unitarians and liberal Jews – support the change in the law and are expected to apply for permission to use their buildings to host same-sex "marriage" ceremonies.
However, the Church of England has pledged not to allow any of its buildings to be used for civil partnership ceremonies, and last year Pope Benedict said same-sex marriage was among the "most insidious and dangerous challenges that today confront the common good".
The proposal also threatens to provoke resistance from politicians, particularly among the Tory right.
Last year, when Parliament considered a move to allow gay "weddings" to be held in churches, a series of senior Tories spoke against.
The former Tory party chairman Lord Tebbit said: "We should be utterly, completely and absolutely clear that a civil partnership is not a marriage, cannot be a marriage, never will be a marriage and should be treated entirely separately from marriage.
"Marriage is celebrated within a church. If we make it a permissive option, sooner or later, the legal proceedings will start to enforce it upon churches against the will of many ministers in those churches."
The amendment, put forward by the gay Labour peer Lord Alli, was added to the former Labour government's Equalities Act. But before this arrangement could be fully legal, ministers had to stage a separate consultation and to pass separate legislation. This is the process to be launched by Mrs Featherstone within days.
Currently, civil partnership ceremonies, which were introduced in 2005, have to be entirely secular and cannot contain any religious element. It is as yet unclear whether the new-style civil partnerships, formalised in a place of worship, would be officially called "marriage" under the law.