Drugs taken by millions of people in the UK for treating depression could also be damaging male fertility, researchers warn.

Two patients given anti-depressants saw their sperm counts fall dramatically while on treatment.

When treatment stopped, the men's fertility was restored. But within days of restarting the drugs, their sperm counts fell again. There were 29 million prescriptions for anti-depressants in 2004.

One in four people suffers a mental illness at some point in their lives.

The researchers, from the Cornell Medical Centre in New York, say that although they had identified only two patients with the problem, the effect was unexpectedly severe, reducing their sperm counts close to zero. The two men had normal sperm counts before treatment started .

The rise and fall in their sperm counts closely followed their starting or stopping treatment, suggesting a clear cause and effect, they say. Similar effects had been seen in a dozen other patients but the link was not as clearly documented.

The men were treated with four different drugs, called selective serotonin re-uptake inhibitors. This is the same class of drugs as Prozac though that was not among the drugs taken.

Peter Schlegel, who led the study, said: "It's a dramatic effect. We need to know how often it happens. We believe that while it has had a profound effect on these two men it could be having a significant but more subtle effect on many more men."

A known side effect of antidepressants is that they delay or prevent ejaculation, but doctors had not suspected that this could affect sperm quality.