It might be that this is involved in giving the sperm's tail beating a kind of turbocharge at the last instant

Dr David Clapham, Howard Hughes Institute

To test their theory, researchers took some mice and genetically altered them so they lacked the gene which helps produce the vital protein.

They found that genetically altered mice were completely infertile, and their sperm were considerably less vigorous.

In a separate test, when the outer layer of an egg, called the zona pellucida, was removed, the altered sperm were able to fertilise it normally.

Researcher David Clapham, from the Howard Hughes Institute, said: "The only thing that seems to be wrong in the sperm of these mice is their inability to penetrate the zona pellucida.

"It might be that this channel is involved in giving the sperm's tail beating a kind of turbocharge at the last instant, when it needs a bit more power to penetrate the zona pellucida."

Unisex drug

A drug to block the action of the protein is now a possibility, he believes.

If this can be achieved, it could be safer than traditional hormonal contraceptives for women.

He said: "Hormonal contraceptives have to be taken daily throughout a woman's fertile life.

"Also, they have potential side effects, such as increased risk of blood clotting and cancer."

He said that, in theory, a drug could also be taken by women, either just before or just after sex.

The research is published in the journal Nature.