The government is facing a union-backed legal challenge over the pension rights of gay workers.
Pensions: Gay couples normally lose out
The unions say new equality laws banning discrimination on the grounds of sexual orientation, due in December, are not being implemented correctly.
They say a loophole will allow pension schemes to continue offering benefits to married couples only.
Religious organisations will also continue to be able to bar gay, lesbian or bisexual people from working for them, unions say.
Some company schemes confer pension rights for gay members, but most do not.
In most cases, if one half of a gay couple dies their pension payments cannot be transferred to the surviving partner.
Gay and lesbian campaigners have long sought to change the rules.
So far seven unions, including Amicus, and Unison, have signed up to the legal challenge.
They want a judicial review into the law change - and have lodged papers with the High Court.
They are arguing that not only does the planned UK law misinterpret a European Union directive, but it may also breach the Human Rights Act.
'Major step forward'
But the government says no such breach has been made.
"The (regulations) are entirely consistent with European Community law and the Human Rights Act. They are a major step forward in combating discrimination at work," the Department of Trade & Industry (DTI) said in a statement.
TUC General Secretary Brendan Barber said: "The Government's new sexual orientation regulations as a whole will make a real difference to gay people, giving them real legal protection at work for the first time.
"But it is unfortunate that the government has decided to exempt those working for religious organisations and wants to bar lesbians and gay men from receiving benefits from certain pension schemes."