A former Archbishop of Canterbury has claimed that legalising gay marriage will pave the way for polygamy and for siblings to wed.
Lord Carey raised the prospect of a union between two women and a man, or two sisters, as he outlined his opposition to the government's plans, which go before the House of Lords on Monday.
He becomes the latest high-profile figure to use incest as an argument the equal marriage bill.
Carey (right) with the Grand Imam of al-Azhar al-Sharif
Critics immediately pointed out that people are banned by law from marrying their relatives, irrespective of gender.
In his article, called 'Love is not enough', Carey claimed that homosexual love and commitment are not enough to warrant marriage - because there is no natural possibility of children.
He wrote: "Once we let go of the exclusivity of a one man-one woman relationship with procreation linking the generations, then why stop there?
"If it is 'about love and commitment' then it is entirely logical to extend marriage to, say, two sisters bringing up children together.
"If it is merely 'about love and commitment' then there is nothing illogical about multiple relationships, such as two women and one man."
He also hit out at the Government, saying it has no moral or democratic right to legalise gay marriage, and describes the legislation as "hasty and ill-considered".
He wrote: "My feeling is that Welby would, in his heart, like to support marriage equality but feels bound by centuries of church tradition and is fearful of the theological earthquake – and likely schism – that would result if he ended the Church’s opposition to the legalisation of civil marriage ceremonies for same-sex couples."