As MPs prepare to debate the government's Civil Partnership Bill campaigners have called for gay marriage to be fully legalised.

The Green Party and gay rights group Outrage are calling for the legislation to be altered to allow same-sex marriage, but MPs will be more concerned with overturning Lords amendments which have threatened to scupper the law when they discuss the issue on Tuesday.

Ministers have put forward proposals to grant formal recognition of long term homosexual partnerships in a bid to tackle discrimination in areas such as inheritance, tax allowances and next of kin status.

Couples who register would have a new legal status as "civil partners" and would, according to the government, "acquire a package of rights and responsibilities".

The scheme will not apply to heterosexual cohabitees on the grounds that they have the option of civil or religious marriage denied to homosexuals.

Attack

But the bill has come under fire from two flanks, with the Greens campaigning for full marriage rights to be allowed.

Outrage chief and party member Peter Tatchell said the current plans were "an affront to democracy and human rights".

"Denying people the right to marry because of their sexual orientation violates the European Convention on Human Rights, which states that every adult person has a right to marry," he argued.

"The Civil Partnership Bill creates a form of sexual apartheid, with one law for heterosexuals and another for gays.

"Same-sex couples are excluded from marriage and opposite-sex partners are excluded from civil partnerships. This is not equality. It reinforces and perpetuates discrimination."

Amendments

On the other hand peers heavily amended the legislation when it passed through the upper house in June.

Conservatives pushed through plans to give groups including carers and family members the same inheritance rights as the new civil partners.

Ministers described the move as "unworkable" and pledged to overturn the changes using the government majority in the Commons while critics accused the Lords of seeking to bring the bill down because of an anti-gay agenda.

However because the bill was introduced in the second chamber the Parliament Act cannot be invoked to enforce the will of MPs, while with the end of the Westminster session approaching, time could be short for the legislation to "ping pong" between the houses in search of a compromise.