A bid to give siblings who live together the same rights gay couples would get under planned new laws has been rejected by MPs.
Germany introduced similar equality legislation in 2001
Tory MP Edward Leigh was jeered as he outlined an amendment to the Civil Partnerships Bill, which gives same-sex couples similar rights to married ones.
Critics said the change would wreck the plans, but Mr Leigh said he just wanted to avoid further injustices.
The House of Lords must now decide whether to reinstate Mr Leigh's plan.
The Tory amendment called for brothers and sisters living together for at least 12 years to get the same rights over property and pensions that are being proposed for gay couples in the Bill.
MPs opposed the move by 74 votes to 381 and the Bill later cleared its final Commons stages.
It returns next week to the Lords, which must decide whether to overturn the MPs' decision and open a battle of wills between the two House of Parliament.
Mr Leigh's proposal is backed by former Tory chairman Lord Tebbit and former shadow minister Gerald Howarth, but is not official Conservative Party policy.
Opening the Report Stage debate, Mr Leigh said: "All we are trying to do is ensure this Bill does not create more injustices.
"I cannot understand, given that the Bill is going to become law, given that homosexual couples are going to have these rights, why people are so violently and strongly opposed to extending these same rights to siblings."
Deputy Minister for Women Jacqui Smith said that it was not the right Bill to address the problems siblings and those living with carers could face.
She said: "Civil partnerships have not been designed as a legal relationship for people that are related to each other.
"It's a legal relationship for same sex couples so that they have that recognition that they currently cannot get."
But Labour's Angela Eagle described the bid as "invidious, divisive and entirely predictable," accusing Mr Leigh of using the Bill to make a point about gay marriage.
Tory frontbencher Alan Duncan said the proposed changes were "wrecking amendments" that created more problems than they addressed.
But fellow Conservative Christopher Chope said he supported the amendment because it give a "wider choice to people who wish to register a co-habiting relationship short of a marriage".
He argued the Bill created a "legal minefield" and was "discriminatory in nature".
"It is effectively a same sex partnership Bill but the government doesn't have the guts to call it that," added Mr Chope.
CIVIL PARTNERSHIP RIGHTS
Social security and pension benefits
Possible parental responsibility for partner's children
Full recognition for life assurance
Responsibility to provide reasonable maintenance for partners and children
Same tax treatment as married couples, including exemptions from inheritance tax on homes
Visiting rights in hospitals
MPs are continuing to debate the Bill and amendments to it.
At the moment gay couples may have no claim on the property they share if their partner dies and they are not registered as a legal owner.
The Bill would give them the same exemption from inheritance tax as married couples have.
A full-page advert backing Mr Leigh's proposal, commissioned by The Christian Institute, appeared in The Times newspaper on Tuesday.
It featured a woman who lived with her sister for 15 years and had to sell the house to pay inheritance tax when she died.
In the advertisement the woman asks: "Why should I have less house-sharing rights than a gay couple?"