Britain can rightly take pride in its tolerant outlook. Indeed, it is this attitude that has helped to build our remarkably successful multi-ethnic society, even as immigration has reached unprecedented levels.

But there is a danger that such tolerance sometimes degenerates into a cowardly reluctance to tackle damaging social practices within certain ethnic minorities. In political circles, the fear of accusations of racial prejudice can create a climate of denial, where hard realities are ignored and difficult truths become unsayable.

That is certainly what happened over the disgraceful sex-grooming scandal in Rotherham, in which the civic authorities, including the police and social services, tried to cover up the systematic abuse of more than 1,400 vulnerable girls by predatory gangs of Pakistani heritage men.

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Baroness Flather, Britain's first Asian woman peer speaks out against marriage among close relatives

Similarly, for too long there was an institutionalised unwillingness to confront the spread of radical extremism. Indeed, in 2007, after Channel 4 made a programme about the influence of hate-preachers in Birmingham mosques, the West Midlands Police urged that the producers be prosecuted for stirring up racial hatred. But it is now time to confront another taboo subject, one that has been hidden from the public, despite the misery it causes to children and families.

I am referring to the outdated, unBritish custom within some Muslim communities of marriage among close relatives, especially first cousins.

This tradition, which is followed mainly by migrant families who originate from the Kashmir region of Pakistan, has the tragic consequence of causing disproportionately high rates of disability among their offspring because of the far greater risks of genetic disorders.

During a debate in the House of Lords this month, I said: 'There is so much disability among the children, which is absolutely appalling. You go to any such family and there will be four or five children, at least one or two of whom will have some disability.' Effectively, we have allowed the import of a medieval convention that should have no place in modern society. The term 'inbreeding' is an unpleasant one, but it is an exact description of what is happening in 21st-century Britain, despite everything we now know about genetics.

The facts are all too stark. A study of the Bradford region found that 37 per cent of babies of Pakistani origin had parents who were first cousins.

Such marriages are partly out of the conformist desire to keep all property within the family, partly out of the wish to bring over another relative to this country.

Outdated, unBritish custom still happens within some Muslim communities

But the heart-rending result is that, according to the Government's own research, British Pakistanis are 13 times more likely than the general population to produce disabled children. The figures are even more shocking in places with a large Muslim population, such as Bradford, where one survey in 2011 revealed that 70 per cent of Pakistani women marry one of their relatives.

It is little wonder, then, that more than 6 per cent of all children born in the city have severe disabilities, including blindness, deafness and neuro-degenerative conditions.

Yet to set out these truths is to invoke the fury of the politically correct brigade, who refuse to consider anything that might intrude on their carefully constructed fantasy of Utopian multi-culturalism.

When I made my contribution in the Lords, arguing that 'it is not fair to the children that they should be allowed to become disabled because of a social practice', I was subjected to a barrage of condemnation.

It's so heart-rending - British Pakistanis are 13 times more likely to have disabled children

One Tory peer, Lord Sheikh, warned that my speech would 'not help community cohesion in this country'. The Muslim Council of Britain went even further, claiming that my 'consistent bigotry has unfortunately forfeited the right to be taken seriously'.

Well, if it is bigoted to be concerned about women's rights and disability, then I wear the insult with pride.

But the Council's argument that I am prejudiced against Muslims is absurd. On the contrary, I have devoted much of my public life to fighting discrimination and supporting ethnic minorities, including Muslims.

I have run schemes to teach English to migrants, as well as carrying out voluntary work with Indian and Pakistani families.

I was born in the city of Lahore, now in Pakistan, and my great grandfather was the famous philanthropist Sir Ganga Ram who is often called the father of Lahore. As the first Asian female councillor in Britain, serving on Windsor and Maidenhead Council from 1976, I fought for local Muslims to be given land to construct a mosque.

But it is precisely this empathy that makes me so disturbed at the incidence of genetic defects caused by first-cousin marriages.

Baroness Flather is disturbed at the incidence of genetic defects caused by first-cousin marriages

As the renowned scientist Professor Steve Jones has said of 'inbreeding' in Islamic communities: 'We should be concerned as there can be a lot of hidden genetic damage and children are much more likely to get two copies of a damaged gene.' Is Professor Jones a 'bigot' for spelling out the reality? Of course not.

Indeed, when he made those remarks in a speech in 2011, he went out of his way to point out that inbreeding was not confined to Muslims, and, historically, had occurred in every part of society, including the Royal Family.

He said: 'We are all more incestuous than we realise. In Northern Ireland lots of people share the same surname, which suggests a high level of inbreeding.

'There's a lot of surname diversity in London, but if you look at the Outer Hebrides there are rather fewer surnames in relation to the number of people.'

Still, he did not escape a backlash from some Muslim groups.

Mohammed Shafiq, chief executive of the Ramadhan Foundation, was reported in the press as saying: 'Obviously, we don't want any children to be born disabled who don't need to be born disabled, so I would advise genetic screening before first cousins marry. But I find Steve Jones's comments unworthy of a professor. Using language like “inbreeding” to describe cousins marrying is completely inappropriate and further demonises Muslims.'

I'm sorry Mr Shafiq takes exception to the word inbreeding and obviously they are not brother and sister - but the fact is they are the next closest relative. The real prejudice is not from those who higlight the problems of cousins marrying. It is from those who hide behind politically correct ideology to protect outmoded practices, thereby harming the lives of tens of thousands of children.

No fewer than one in ten children of first-cousin marriages either dies in infancy or develops serious, life-threatening disabilities. British Pakistani parents are responsible for only about three per cent of all births in the UK, yet account for a third of all the babies born with genetic defects.

Other societies have long been aware of the health dangers from marriages between close relatives. In Greece, for instance, there used to be a major problem with the inherited blood disorder thalassemia, which can lead to bone deformities and cardiovascular disease.

One of the causes of its high prevalence in Greece - particularly among the Greek islands - was inbreeding among isolated rural communities. But the country completely eradicated the disease through rigorous testing.

Close ties In 1875, 3.5 per cent of middle-class married couples in England were cousins, according to research by Charles Darwin's son, George Advertisement

This ensured that adults with the thalassemia gene - who may not have been physically affected by the disorder themselves - did not try to reproduce with other carriers.

Unfortunately, because of the practice of marriage among relatives, thalassemia is still present in the Indian subcontinent and among Pakistani communities here. That is profoundly cruel to the children, who have to undergo regular, exhausting blood transfusions.

It is also a tremendous burden on the NHS, which is already overstretched, and to the taxpayers who have to provide the funds for this healthcare.

What is extraordinary is that so many Pakistani families do not seem to care, either about the potential disability in their children or about the cost to the health service.

Knowledge about genetics has never been deeper, yet such families appear to remain trapped in ignorance or indifference. That has to change.

It may not be possible legally to ban first-cousin marriages, but the Government could embark on education and awareness-raising campaigns to alter attitudes among Muslims, making such arrangements socially unacceptable.

One very practical step could also immediately be taken, and that is to require all couples proposing a first-cousin marriage to undergo genetic screening. If the genetic history of either partner highlights any serious health risks, then the marriage should not proceed.

And for partners coming to Britain from Pakistan, the British High Commission could arrange for genetic tests to be carried out at a number of local reputable clinics in that country.

But ultimately we have to push for an end to this practice. By continuing to accept these abuses, the British state colludes with suffering, imposes a heavy burden on itself and undermines its own liberal principles.