There are still some real concerns about the effects of environmental oestrogen on male fertility

Mr Richard Kennedy, British Fertility Society

But doctors say it shows there is no immediate need for the "near-panic" about the decline in male potency, blamed by some on environmental pollution.

The research team measured the average time it took couples wanting a child to conceive. This, they say, is a sensitive way of measuring the fertility of the nation.

They found that between 1961 and 1965, 65% of couples managed to conceive within six months.

In the years between 1991 and 1993, this had risen to 72%.

The trend was has been steadily upwards over the last 40 years, with the exception of slight falls in the late 1970s and late 1980s.

The study's authors say in the Lancet medical journal that neither social trends, nor medical advances since the 1960s can explain the change.

In fact, decisions by today's parents to have children later in life would be more likely to decrease fertility rather than improve it.

And improvements in infertility techniques over the decades would simply have added more people who conceived after a long wait to the research.

No panic

tThe report says: "If a decline in male fertility has taken place, it has been more than compensated for by a countervailing increase in couple fertility."

Mr Richard Kennedy, a consultant obstetrician and gynaecologist at Walsgrave Hospital near Coventry, and honorary secretary of the British Fertility Society, said that while there were threats to the future fertility of the UK, there was no need to panic.

He said: "There are many factors which impact on fertility. We are able to help many more patients with fertility problems than we could in the 1960s.

"And a rise in sexually-transmitted diseases could also lead to a rise in fertility problems for women."

He said that it was difficult to compare sperm measurements taken decades ago to those taken more recently, as technology had changed.

However, he added: "There are still some real concerns about the effects of environmental oestrogen on male fertility.

"These need to be evaluated."