A major medical row will erupt this month when scientists and health experts hold two key meetings to discuss the controversial subject of marriages between cousins and their impact on health in Britain.

The debates will be held by the Royal Society of Medicine as part of its 100 Years of Medical Genetics celebrations on 23 May, and by the Progress Educational Trust at Clifford Chance in east London on 29 May. Both will reveal deep divisions among scientists.

Some researchers and politicians say inter-cousin unions, which are highly prevalent among British Pakistanis, have led to a striking rise in the incidence of rare recessive disorders, many of them fatal, in areas such as Bradford. The trend has led to calls for cousin marriages to be banned.

'In our local school for deaf children, half the pupils are of Asian origin though Asians only form about 20 per cent of the population,' said Ann Cryer, MP for Keighley. 'I also know of several sets of parents in my constituency who are cousins and whose children are severely disabled. I have no doubt that the mothers and fathers being closely related to each is a key factor.

'We give warnings about the dangers of smoking, drinking and taking drugs. It is now time that primary care trusts started doing the same for cousin marriages.

Others who have warned about the dangers include Phil Woolas, an environment minister, who provoked fury earlier this year when he said the culture of arranged marriages between first cousins was the 'elephant in the room' and argued that if 'you have a child with your cousin , the likelihood is there will be a genetic problem'.

This last claim is hotly disputed by genetic counsellors and Muslim doctors. They point out that the danger of a child having birth defects if the parents are cousins is double that of other children, which means the risk rises from about 2 per cent in the general population to about 4 per cent when the parents are closely related. A risk of 4 per cent therefore does not make it 'likely' there will a genetic problem, as Woolas claimed, say genetic counsellors.

'The danger posed by cousin marriage is highly exaggerated,' said Aamra Darr, a senior research fellow at Bradford University. As Darr pointed out, women in Britain are more likely now to have children when they are over 30, increasing the likelihood of them having babies with Down's Syndrome. But no one suggests that there should be a ban on over-30s having babies, Darr added. Medicine has adapted to improve screening services for these women.

'We should recognise that for British Pakistanis, cousin marriages represent significant cultural advantages. Recent advances mean we can pinpoint many of those at risk of having affected babies. That is where we should be placing our efforts.'

Meanwhile, as geneticist Alan Bittles of Murdoch University, Perth, has pointed out, a large majority of children born to first cousins are healthy: 'Many famous figures married their first cousin, including Charles Darwin and Albert Einstein.'