Fertility specialists revealed new evidence yesterday that male fertility declines with age as they warned that men's lack of awareness of their own biology may be playing a part in low birthrates.

The warning comes as a Guardian/ICM poll highlights a shift in attitudes towards childbearing in Britain, in which low birthrates are blamed by many couples on a desire to further their careers and live comfortably. The poll revealed that while the vast majority of people understood the importance of a woman's age in achieving a successful pregnancy, many couples started trying for a family later in life regardless.

In the latest research, IVF experts studied the often neglected question of whether a man's age affects the chances of a couple having children. They concluded that while the age of 35 should be considered the "amber light" in a woman's reproductive life, men reach a similar milestone at 40.

The researchers, from fertility clinics across France, examined rates of successful pregnancies in 1,938 couples from 59 clinics. The study showed that a woman younger than 30 was 25% less likely to conceive if her male partner was 40 or older than if he was of similar age. And if the woman was 35 to 37, she was twice as likely not to conceive as to have a successful pregnancy if her partner was 40 or older.

Writing in the journal Fertility and Sterility, Elise de La Rochebrochard, who led the study at the French National Institute for Demographic Studies, said: "Our results provide, for the first time, strong evidence for a paternal age effect on failure to conceive that is linked only to biological male ageing." She added: "In reproduction, age must no longer be considered as the concern of the woman, but as that of the couple. Similar to maternal age over 35 years, paternal age over 40 years is a key risk factor for reproduction."

Allan Pacey, the secretary of the British Fertility Society and senior lecturer at Sheffield University, said the evidence for male fertility dropping with age was beginning to stack up. A previous study had revealed a greater risk of Down's syndrome in children born to older fathers.

"Women seem to be more attuned to their biological clocks," he said. "With men the clock seems to tick more slowly, but often I don't think they hear it at all."

Dr Pacey said the government should tackle the problem by broadening sexual education to include advice on starting families. "What we're very good at in schools is getting the message across about sexually transmitted diseases and early pregnancy, but what we're not so good at is explaining when is the best time to have family."