The Bradford research found that up to six out of 100 babies born to parents in the city who are blood relations have birth defects, compared with three in 100 of those who have not inter-married.

The Bradford rate is more than three times the national average of just under two per cent.
The findings are likely to renew controversy over what many claim is a taboo subject because of the cultural and religious issues relating to first-cousin marriages.

Ann Cryer, Labour MP for Keighley during the last Parliament, condemned cousin marriages, saying they harmed children and were arranged to keep wealth and property within families.

She said they put a strain on the NHS in areas with big Pakistani communities because many children needed treatment for often disabling genetic disorders.

Similarly, Phil Woolas, environment minister at the time, said the genetic problems had to be addressed as a ‘matter of public health’ by anyone who was supportive of the Asian community.

The new study collected data on 11,300 babies involved in the Born in Bradford project, a long-term study following the health of babies born in the city between 2007 and 2011.

Of these, 45 per cent were Pakistani and 40 per cent white British. Previous research has suggested a higher risk of health problems for babies born as a result of consanguinity – marriage to a blood relation – including studies involving Jews, Arabs and Romany gipsies.

But this is the biggest so far, and its size allowed researchers to rule out an effect caused by deprivation. Study co-author Professor Neil Small, from the University of Bradford, said it would be wrong to use the findings to dissuade blood relatives from marrying.

‘But we do want to ensure that couples are aware of any risks so that they can make informed choices when planning their families,’ he added.