More than 2.5 million men in Britain may be suffering from fertility problems, with high rates of smoking, stress and drinking believed to be contributory factors, a report has found. But many men are in denial about the risk of infertility and tend to believe that problems in conceiving are more likely to be the "fault" of the woman.

A survey of GPs and male patients for Norwich Union Healthcare found that while doctors were worried about the extent of the problem, many men were unaware that they could have problems fathering a child.

More than 30 per cent of the GPs said they were concerned about the decline in male fertility. More than a quarter said that low male fertility could have a detrimental impact on the population, unless men changed their lifestyles.

Asked to name the biggest causes of male infertility, GPs listed smoking, excess alcohol consumption and stress. Research for the survey also suggested that 9 per cent of the male population in Britain could be suffering from low fertility.

The quality and number of sperm that men produce has declined in the past 30 years, with male infertility accounting for about a third of couples' problems conceiving.

But when almost 1,000 men were questioned for the report, only 12 per cent said they were concerned about their own fertility, and only 5 per cent were aware that they could be suffering from a low sperm count. More than half - 59 per cent - believed women were more likely to suffer from fertility problems.

One in three men believe their partners will be able to conceive within two months of trying, while three-quarters of GPs said they expect an average couple to take between three and six months to achieve a pregnancy.

Despite the lack of awareness, 12 per cent of men who had children said it had taken longer than they anticipated for their partners to conceive.

Dr Ann Robinson, a GP and spokeswoman for the report, said: "The results of this survey are shocking and should be a wake-up call to men and women that drinking and smoking too much ... can affect your ability to start a family."

Doctors called for boys to be given school lessons about fertility problems and said men should be given fertility "MoTs".