Men who keep their phone in their trouser pocket are being warned it could damage their chances of becoming a father.

A new study has found that keeping a mobile too close to certain part of the body can risk 'cooking sperm'.

The study - which was carried out in Israel, and published in the medical journal Reproductive BioMedicine Online - also found that talking on a mobile for more than an hour a day or while a phone is charging is even more dangerous, and could double the likelihood of lower sperm concentration.

The study says male infertility constitutes 30% to 40% of all infertility cases.

It investigated an association between characteristics of mobile phone usage and semen quality.

Some 106 men were referred for semen analysis as part of the study.

The results were analysed according to criteria from the World Health Organisation (WHO).

Among men who reported holding their phones less than 50 cms from their groin, a higher rate of abnormal sperm concentration was found - 47.1% versus 11.1%.

While analysis also revealed that talking while charging the device was a risk factor for abnormal sperm concentration.

"Our findings suggest that certain aspects of cell phone usage may bear adverse effects on sperm concentration. Investigation using large-scale studies is thus needed", it says.

Some studies have shown a continuous decline in semen quality since the beginning of the 20th century.

One assumed contributing factor is radio frequency electromagnetic radiation, which is emitted from mobile phones.

Dr John Kennedy from SIMS fertility clinic in Dublin says phones should be kept in a jacket pocket instead.