The biological clock is ticking not just for women in their 30s. Men of a similare age also need to worry about declining fertility, according to research published today.

American scientists have discovered that genetic damage to sperm starts to cause infertility in men as young as 35, raising concern that those who delay fatherhood for their career could be taking an irreversible risk.

The findings, to be presented at the American Society of Reproductive Medicine's annual conference in Seattle, appear to be the strongest evidence yet for a significant drop in male fertility in the late 30s.

Researchers from the University of Washington looked at sperm quality in 60 men aged between 22 and 60. All thad healthy sperm counts. The group was then divided in half - one for those aged 35 and over and the other for those under 35.The team, led by Dr Narendra Singh, found that the older group had higher concentrations of sperm with broken strands of DNA, and that such genetic damage was more acute in the older group.

In addition, their immune systems were much less efficient at finding and destroying faulty sperm by a process of programmed sperm suicide known as apoptosis.

The sperm of the older men were also less active so they had less chance of fertilising an egg.

"This study clearly demonstrates an increase in sperm DNA breaks with age. For the first time, an age related decrease in sperm apoptosis is found," said Dr Singh.

The research is a blow to traditional assumptions that, if a couple fail to conceive, the problem is due to the woman. Recent research suggests 40% of problems are attributable to women, another 40% to men, 10% to the partners combined and the remaining 10% to unexplained causes.

William Keye, the president of the American Society of Reproductive Medicines, said that men concerned about their fertility should avoid smoking and drinking.

"While there's nothing anyone can do about getting older, men who want to retain their best capacity to father children should try to minimize contact with toxic agents and maintain a healthy lifestyle," he added.