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Cousin marriages are a contentious topic in the UK - but, surprisingly, it is not illegal.

A new BBC Three documentary, available on iPlayer right now, is seeking to throw light on the subject.

There has been a great deal of debate in the past few years in Britain about whether to discourage cousin marriages through government public relations campaigns or ban them entirely.

The debate has been prompted by a Pakistani immigrant population making up around 1.5 per cent of the British population.

Just under a decade ago, then-Environment Minister Phil Woolas sparked controversy.

Mr Woolas said in 2008: "If you have a child with your cousin the likelihood is there'll be a genetic problem".

He also claimed such marriages were the "elephant in the room".

After this, a prominent physician, named Mohammad Walji, also spoke out against the practice, saying that it is a "very significant" cause of infant death, and his practice has produced leaflets warning against it.

However, Alan Bittles of the Centre for Comparative Genomics in Australia states that the risk of birth defects rises from roughly two per cent in the general population to four per cent for first cousins and therefore that "it would be a mistake to ban it".

First-cousin marriages were once quite common in Europe, especially among the elite – Charles Darwin married his first cousin Emma Wedgwood.

But all this changed in the late 19th-century.

As social mobility grew, so did a perception of the risks associated with cousin marriages.

The stigma was supported by early studies into human genetics suggesting that "recessive" versions of a gene are more likely to be expressed in the children of genetically related parents, as well as more likely to be defective.

While most states in the UK have outlawed the practice, Professor Spencer, an evolutionary zoologist, said these laws should be repealed.

He claims: "Neither the scientific nor social assumptions behind such legislation stand up to close scrutiny. Such legislation reflects outmoded prejudices about immigrants and the rural poor and relies on over-simplified views of heredity. There is no scientific grounding for it."

Peter Corry of St Luke's Hospital in Bradford estimates that among people of Pakistani descent in the city, 55 per cent of whom marry first cousins, the risk of recessive genetic disorders is between 10 and 15 times higher than in the general population.

But this needs further analysis, he added.