They say research could eventually pave the way for halting the advance of Motor Neurone Disease with treatments.

However, there is no evidence that, whatever its origins, the disease is contagious.

The condition is currently incurable, and causes gradually increasing disability, while leaving the mind undamaged.

We think this knowledge will help us finally uncover what causes this disease and may someday lead us to developing a treatment

Martina Berger, researcher

Sufferers - while perfectly conscious - are trapped in their bodies, unable to speak or move.

Famous people with MND include physicist Professor Stephen Hawking, who uses a computer generated voice to communicate.

He is unusual among MND sufferers in that he has survived with the condition for three decades - most die within five years.

Approximately 5,000 people in the UK are thought to have the condition in some form.

Researchers had long suspected that MND, also known as amyotrophic lateral sclerosis, or Lou Gehrig's disease after the US baseball player who died of it, was caused by viral infection.

However, a team at the University of California at Irvine, working with the Rockefeller University in Lyon, claim to have found the strongest links yet.

Virus found in sufferers

They examined the bodies of 17 people who had died from MND, and found that 15 of them had the virus present in their spinal cords.

The virus, similar to a common virus called echovirus 7, was found in only one of 29 people who had died from other causes.

Echovirus 7 is normally linked with meningitis or encephalitis.

Martina Berger, a researcher at the University of California, said: "In this study we were able to identify a virus known for nerve damage in the exact areas of the nervous system that are affected by the disease.

"We think this knowledge will help us finally uncover what causes this disease and may someday lead us to developing a treatment."

However, a spokesman for the Motor Neurone Disease Association said that it was not yet clear whether the virus caused the condition, or whether motor neurone disease itself made it easier for the virus to enter the cord.

"We don't know whether the virus is the cause, or just an effect, of the disease," she said.

MND is caused by nerve cells in the spinal cord which control muscle movements becoming damaged, gradually weakening every muscle towards paralysis.

Even those which control swallowing eventually fail, meaning patients have to be artificially fed, and death is finally caused by the failure of key muscles which control breathing.

Although a few motor neurone cases could have a heridatary cause, most at present appear at random.

The research was reported in the medical journal Neurology.