A government minister has insisted he was facing up to the "elephant in the room" when he raised concerns about health defects related to cousin marriage.
But critics have called for Phil Woolas to resign as environment minister, insisting his comments "verged on Islamophobia".
Mr Woolas, MP for Oldham East and Saddleworth, used an interview with a Sunday paper to argue that the trend for cousin marriage among some quarters of Britain's Asian population was leading to birth defects.
He told the Sunday Times: "If you have a child with your cousin the likelihood is there'll be a genetic problem.
"The issue we need to debate is first cousin marriages, whereby a lot of arranged marriages are with first cousins, and that produces lots of genetic problems in terms of disability [in children]."
He said the issue had been difficult to address because of cultural sensitivities but insisted it was a cultural, not religious issue.
Mr Woolas continued: "If you talk to any primary care worker they will tell you that levels of disability among the... Pakistani population are higher than the general population. And everybody knows it's caused by first cousin marriage.
"Awareness does need to be raised but we are very aware of the sensitivities."
It is thought cousin marriage, which is legal in the UK, is most prevalent among families originating from rural Pakistan.
A Newsnight poll in 2005 found 55 per cent of Pakistanis in Britain marry a cousin and are 13 times more likely to produce a child with a genetic disorder.
The Muslim Public Affairs Committee (MPAC) attacked Mr Woolas' comments and called on Gordon Brown to "sack him".
MPAC spokesman Asghar Bukhair said his comments had "verged on Islamophobia".
Speaking to BBC News, he accused the environment minister of commenting outside his brief and ignoring the links between pollution and birth defects.
But Labour MP Ann Cryer defended her colleague. The MP for Keighley claimed the health implications were visible to anyone visiting a paediatric ward in her constituency.
Fellow minister Geoff Hoon also defended Mr Woolas, telling Sky News there was a need for expert analysis to discover the full extent of the problem.