Plans to give legal status to gay and lesbian couples discriminate against heterosexual couples, campaigners have said.
Gay couples will have property and pension rights
Under the plans, to be outlined by the government on Monday, gay and lesbian couples will be awarded the same pension and property rights as married couples.
Their relationship will be recognised as a "civil partnership", as long as they officially register by signing a document.
The moves will give next-of-kin rights in hospitals, allow gays to benefit from a dead partner's pension and exempt them from inheritance tax on a partner's home.
However, the changes have been criticised by human rights campaigners who complain that heterosexual non-married couples are discriminated against.
Veteran gay rights campaigner Peter Tatchell described the plans as "heterophobic" and "retrograde".
"It is divisive, heterophobic and discriminatory to exclude unmarried heterosexual couples," he said.
"Cohabiting heterosexuals also lack legal recognition and protection. This is a grave injustice."
He added: "It is a pity the government has opted for an unimaginative, watered down version of marriage, instead of having the foresight to devise an entirely new, modern legal framework for partnership recognition."
Liberal Democrat equality spokesman Evan Harris also criticised the government for ignoring unmarried heterosexual partners while welcoming the recognition for same sex couples.
"It is typical that the government has only done the bare minimum," he said.
"The decision to exclude opposite sex couples from claiming the rights conferred by civil partnerships will be a bitter disappointment to hundreds of thousands of heterosexual unmarried couples.
"Currently the government treat them as married when cutting their benefits, but ministers are clearly refusing to reciprocate when it comes to pension sharing."
But Minister for women and equality Jaqui Smith denied the plan was discriminatory.
"There is already a legal opportunity for heterosexual couples through marriage to have their relationship recognised," she said.
"There is no legal opportunity at all for same sex couples. We think that's wrong and that's what we're putting right today."
Gay rights group Stonewall was "delighted" with the plan.
Chief executive Ben Summerskill said: "It's not just social status that matters, like the right to visit each other in hospital, but the right to share a partner's pension, for example."
He said that was "something available to every heterosexual".
The BBC's Kim Catcheside says the move will not find favour in some religious circles, with the Church of England split in its opinion of the issue.
"These proposals will only exacerbate these divisions," she said.
A consultation document on the plans will be published by deputy minister for women and equality at the
Department of Trade and Industry, Jacqui Smith.