Controversial Euro MP Roger Helmer has criticised moves to legalise gay marriage, saying: "If two men have a right to marry, how can we deny the same right to two siblings?"

Civil partnerships have been available under UK law since 2004 and give two people the same rights as marriage. However, the two processes are technically different.

Prime Minister David Cameron has said that he wants to legalise gay marriage in the UK, but Britain's most senior Catholic, Cardinal Keith O'Brien, said the move would go against "natural law".

On his blog yesterday, Mr Helmer, who lives in Ashby Magna and represents the East Midlands, wrote: "Cardinal O'Brien makes the point, quite reasonably, that once you start to tamper with the institution of marriage, you get into some very murky water indeed.

"If two men can be married, why not three men? Or a two men and a woman? He could have gone further. Why not a commune? If two men have a right to marry, how can we deny the same right to two siblings? Are we to authorise incest?"

Responding to Cardinal O'Brien's claim that plans to legalise gay marriage were "a grotesque subversion of a universally accepted human right", Mr Helmer wrote: "Brilliant. I wish I'd thought of the phrase. Perhaps he ought to be a politician."

It comes just a few days after Mr Helmer defected from the Conservatives to join the UK Independence Party, saying that David Cameron was "too left wing".

However, his comments on gay marriage immediately came in for criticism.

Canon Barry Naylor, urban canon at Leicester Cathedral, said: "Comments like that are clearly intended to be offensive.

"I rarely agree with what David Cameron has to say, but I believe his proposal to legalise same-sex marriage would actually strengthen the institution of marriage, rather than weaken it."

Peter Adams, 65, who lives in Leicester city centre, and works with gay and lesbian people in the area, said: "Mr Helmer's comments are ridiculous. It's such an unintelligent remark to make that you can't give it any credence. There's no logical step between same-sex marriage and incest."

When asked by the Mercury whether his comments were offensive, Mr Helmer said: "I don't think so. I've nothing against homosexuals, they can do what they like.

"But marriage has been recognised for thousands of years as a bond as being between a man and a woman. It's the best environment in which to raise children."

The Very Rev Vivienne Faull, Dean of Leicester, said: "Our position is quite clear; the Church recognises arrangements made by the state for two people of the same sex to be in a civil partnership.

"However, the Church itself only recognises marriage as between a man and woman exclusively.

"The only way to change this is through the General Synod of the Church of England.

"However, a debate on this issue has not been scheduled yet."

WHAT’S THE DIFFERENCE? Marriages and civil partnerships

Legally, civil partners must be treated the same as married couples on a wide range of legal matters such as inheritance tax.

However a same-sex couple who have had a civil partnership cannot currently claim to be married.

Campaigners, such as Peter Tatchell, say that having a two-tier system for recognising the partnership between two people is a form of “apartheid”.

The Government could overhaul the law on marriage itself, allowing same-sex couples to have civil weddings and declare themselves “married”, rather than simply in a civil partnership.

Any proposal to introduce same-sex marriages would refer to civil marriages, in register offices and other licensed venues, rather than church weddings.

No church or religious group could be forced to host a civil partnership ceremony against their will under the Equality Act.