Men who watch more than 20 hours of television a week risk halving their sperm count, researchers warn.

A sedentary lifestyle can have a major impact on a man’s ability to reproduce, the research found.

And while regular, vigorous exercise was shown to boost sperm count, excessive television-watching can counteract the positive effects of physical activity.

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The study, published by the British Journal of Sports Medicine, looked at the lifestyles of 189 healthy men between the ages of 18 and 22, over a three-month period, to establish a link between environmental factors and semen quality.

It found an increasingly idle lifestyle might be a contributing factor to declining sperm levels. Other factors assessed included medical or reproductive health problems, diet, stress levels and smoking.

Men who watched more than 20 hours of television a week had a sperm count 44 per cent lower than those who watched the least, it found.

Volunteers who were most physically active, doing more than 15 hours of exercise a week, had a 73 per cent higher sperm count than the least physically active.

However, men who did regular exercise but also watched a lot of TV recorded lower sperm counts.

The study has bolstered concerns that sperm counts and quality in the Western world have deteriorated over the past few decades. In December, the world’s largest investigation into this area found quality and concentration had fallen sharply. Between 1989 and 2005, average sperm counts fell by a third according to analysis of 26,000 men.

A worldwide drop in sperm count levels has also been accompanied by a rise in testicular cancer – rates have doubled in the last 30 years – and in other male sexual disorders such as undescended testes. A separate investigation last year challenged guidance from the National Institute for Health and Clinical Excellence, which suggests smoking and drinking can have an adverse effect on sperm stocks.

The study by researchers at the University of Sheffield and Manchester compared the lifestyles of 939 men with poor sperm quality with 1,310 men with normal sperm quality. It found “little evidence” that a high BMI, excessive alcohol consumption or recreational drugs were contributing factors to sperm quality.

It also found that wearing boxer shorts rather than tighter underwear was linked to higher sperm levels.

Dr George Chavarro from the Department of Nutrition at Harvard School at Public Health, senior author of the recent study, said: “In general, very little is known about what influences sperm count.”

There was even evidence that high levels of physical activity might have a detrimental effect on quality and quantity.

“I was sceptical of the results of the study on athletes because they are not representative of normal people, because of the high levels of intensity that these people achieve,” Dr Chavarro adds.

“One of the few things we do know is that obesity lowers sperm count – the idea that a more sedentary lifestyle speaks to that.”

Health tips: boost sperm count

Keep cool Looser underwear raises body temperature so opt for boxer shorts over Y-fronts

Eat more seafood Oysters in particular contain zinc, which plays a role in “activating” sperm

Go organic Some pesticides found in food can stop testosterone production for up to 17 hours

Eat nuts Studies suggest walnuts, right, improve the vitality and motility of sperm

Drink less Excessive alcohol consumption may lower testosterone levels by up to 20 per cent