A key gene mutation that more than doubles the risk of asthma, hay fever and eczema has been identified by scientists.

The filaggrin gene helps skin produce a protective barrier against the foreign bodies that trigger allergies.

An analysis of 24 previous studies has shown people with defects in this gene are much more likely to develop unwelcome reactions such as sneezing and itchy eyes, severe dermatitis, asthma, hay fever and eczema.

The gene mutations also increased the risk of asthma in people with severe dermatitis, according to the findings published online in the British Medical Journal.

Health scientist Professor Aziz Sheikh, of the University of Edinburgh, said: "What we found was a more than doubling in the risk of these allergies among people with this defect.

"Allergies are becoming a huge problem, particularly in Britain which is top of the league table. It could be environmental factors, including the frequency of bathing and use of soaps and detergents, that have contributed to the rise.

"What's quite striking is the very high proportion of people who are getting eczema, it's an incredibly common disorder, and if the filaggrin gene is the major factor. It may be we can target it with drugs that can repair this protective skin barrier in due course.

"Why eczema is important is increasingly we think eczema is a herald condition for individuals to go on to develop other allergic conditions, such as asthma and hay fever."

Many children grow out of the disease, but some will continue to suffer for the rest of their lives.

Prof Sheikh, who advises the Government on allergy issues, said: "These findings provide strong supporting evidence that, at least in a subset of those with allergic problems, the filaggrin gene defect may be the

fundamental predisposing factor not only for the development of eczema but also for initial sensitization and progression of allergic disease.

"Our finding suggests filaggrin is a robust biomarker for allergic conditions."

Allergic responses are caused when the immune system wrongly identifies allergens such as dust mites, pollen, peanuts or cat hair as being dangerous.

This triggers the release of histamine in the body, which causes minor symptoms such as itching, wheezing, and sneezing but can also prove fatal in the most serious cases.

The findings could lead to new therapies that prevent allergies reaching epidemic proportions, by stopping the immune system overreacting in this way.

The four main environmental factors thought to be causing a rise in allergies are the exposure to infectious diseases during early childhood, pollution, general levels of allergens and dietary changes.

About one in three children in Britain and other westernized nations suffer from allergies.

Prof Sheikh said: "We call it the allergy march. It is an enormous problem and it is getting bigger all the time but in the future we hope we will be able to screen children at an early age for this gene defect and then start them on a program of treatment."

Eczema can range from a slightly irritating patch of sore skin to more extreme cases where extensive areas may become inflamed and unbearably itchy, known as severe dermatitis or 'atopic' eczema.

Children with the condition are more likely to subsequently develop other allergic conditions, in particular asthma, but also hay fever and others.

The filaggrin gene defects are carried by more than 10 per cent of the UK population.

Pediatrician Prof Hugo Van Bever, of the National University Singapore, said: "This study represents an important breakthrough in understanding the genetic basis of this complex disorder.

"The next challenge will be to distinguish different genotypes of allergy, which could revolutionize the prevention, diagnosis, and treatment of allergy in children."