Men who pile on pounds cut their chance of being a father

Nor could it be explained by the overweight and obese men simply having sex less often than their slimmer counterparts, the journal Human Reproduction reports.

The men classed as obese had a body mass index or BMI of 30 or above, while the overweight men had a BMI of between 25 and 29.9. A BMI of between 20 and 24.9 is generally classed as normal.

The researchers from the National Institutes of Health in the US found underweight men with a BMI below 20 also suffered from fertility problems, but they only accounted for one per cent of the men studied so the statistics were not reliable.

It is thought the obesity may affect fertility by lowering levels of the male sex hormone testosterone.

The researchers concluded: "Increased adiposity (fat) could produce biologic changes in men that reduce their fertility.

"If such changes occur, and are reversible, weight loss may improve their chances of conception."

Other research has shown that obese men tend to have poorer quality sperm.

Doctors fear that infertility may be becoming more of a male than a female problem, with fertility clinics treating more and more cases of male infertility.

It is thought that rising levels of pollution may be harming sperm quality.

Alternatively, it may simply be that more men are putting off fatherhood until later in life, with men over 40 making up around a quarter of consultations at fertility clinics.

Although infertility can also be linked to being thinner than normal, the scale of the obesity problem means that infertility linked to being overweight is more likely to have an impact on the population.

• Half of all boys and a fifth of girls aged between six and ten will be obese by 2050.

The alarming prediction - contained in a Government report - yesterday fuelled concerns the nation is facing a health timebomb.

Experts say unless urgent action is taken, an entire generation faces an old age blighted by heart disease, cancer and diabetes.

Tam Fry, of the National Obesity Forum, said: "Unless we take proper steps to tackle it, we are facing disaster in the near future."

The data comes amid fears that modern technology, rising traffic levels and safety concerns have created a stay-indoors generation.

The report authors warn the Government will not be able to meet its target of halting the rise in child obesity by 2010.

Research leader Klim McPherson, of Oxford University, said the 2050 predictions for children aged 11 to 15 differed to the younger age group, with 23 per cent of boys and 37 per cent of girls expected to be obese.

Currently, around one in ten under-tens and one in four 11 to 15-year-olds is classed as obese.