Fears about the declining levels of male fertility have been raised by new research which reveals sperm counts dropped by almost a third in little more than a decade.

The average sperm count plummeted by 29 per cent between 1989 and 2002, a study at the Aberdeen Fertility Centre in Scotland shows. The findings of the research prompted immediate calls for further investigation into the factors that influence male fertility.

Dr Siladitya Bhattacharya, who led the research, said: "It highlights the need for research to discover what the reasons might be as well as the need for further studies elsewhere in the country to see if they support our results."

Previous research recorded sperm counts falling around the world by an estimated 50 per cent in the past half-century, but scientists have not established the full extent of external influences that may affect a decline in the quality of sperm. Drug use, alcohol, smoking and obesity are among the factors most frequently blamed. The effects of the environment, including the presence of pesticides, chemicals and radioactive material, have also been linked to decreases in fertility.

The preliminary findings of the research will be presented today to the Association of Clinical Embryologists and the British Fertility Society at a conference in Liverpool.

The study, entitled Semen Parameters in the Northeast, was based on an analysis of 16,000 semen samples from men who attended the Aberdeen Fertility Centre, the only such clinic in the Grampian region.

There has been speculation that the low sperm levels may be a consequence of fallout from the Chernobyl nuclear disaster in 1986. Rainfall as the radioactive dustcloud passed over eastern Scotland may have exposed people in the area to unusually high amounts of radiation. But Dr Bhattacharya said it was "almost impossible" to confirm or deny precise factors.