The discovery could lead to significant advances in infertility treatment.

The key is a protein called dynein, which is also found in the lungs, the nervous system and elsewhere in the body.

The scientists, from Leeds University, have taken the first photographs of individual molecules of dynein.

Dynein molecules are attached to tiny tubes within the sperm tail Dr Peter Knight

And this has helped them to work out how the protein creates movement.

Project leader Dr Peter Knight said: "Dynein molecules are attached to tiny tubes within the sperm tail, and as the molecules change shape, they make the tubes slide back and forth causing the sperm's tail to wriggle.

"We call these proteins molecular motors, as they work very much like engines, using fuel which the body creates from the food we eat to power movement."

The scientists identified this movement by combining thousands of electron microscope images of individual molecules of the protein under different biochemical conditions - the first time such images had been obtained.

Lead researcher Dr Stan Burgess said: "Combining the images revealed the major shape change within the dynein molecule.

"That change is equivalent to the piston movement in a car's engine which ultimately drives the wheels round."

Tiny hairs

As well as making sperm wriggle, dynein also powers the tiny hairs (cilia) which push eggs along the Fallopian tubes towards the womb.

The hard-working protein also helps clear the lungs of airborne debris and transports chemicals within our nervous system.

Head of the University's molecular contractility group, Professor John Trinick said the molecular motors were similar to a railway network.

"Our body is full of proteins which form tracks. Along these tracks, molecular motors are the locomotives, transporting a variety of cargoes to wherever they are needed."

The protein plays a crucial role in a variety of gynaecological, bronchial, fertility and neurological disorders.

It also ensures the body's internal organs are the right way round: the body's left-right axis, which places the heart on the left and the liver on the right, is caused by the activity of cilia during early development of the embryo.

The research is published in the journal Nature.