Smokers are at greater risk of developing multiple sclerosis, according to a study that has established the first clear link between smoking and the nerve disease.

Scientists found that smokers in their forties were almost twice as likely as non-smokers to develop MS in later life, with male smokers having 2.7 times the risk.

The study examined 87 MS patients in a sample of 22,312 people between the ages of 40 and 47 in the Norwegian county of Hordaland, in order to identify the environmental factors that increase the chances of developing the disease. Professor Trond Riise, of the University of Bergen, said: "This is the first time that smoking has been established as a risk factor ... Hopefully, these results will help us to learn more about what causes MS by looking at how smoking affects the onset of disease."

Multiple sclerosis is an auto-immune disease brought about when the body's immune defences attack the fatty sheath of material that insulates and protects the nerve cells of the brain and spinal cord. Patients can suffer stiffness, spasticity, pain and general fatigue. The causes of the disease are thought to be a combination of genetic and environmental factors, such as viral infections that damage the central nervous system.

Professor Riise said the link with cigarettes might be because smokers are more prone to viral infections of the throat. Smoking is known to weaken the important tissue barrier between the blood and the brain, perhaps increasing the risk of viruses affecting the central nervous system. "There are several plausible mechanisms that could explain why smokers are at significantly higher risk than non-smokers. Smoking is known, for instance, to interfere generally with the immune system. Or if could be the direct toxic effects of nicotine," he said.

In addition to heart disease and lung cancer, smoking has already been linked with other diseases that are caused by a defective immune system, such as rheumatoid arthritis.

Mike O'Donovan, chief executive of the MS Society, said the findings needed to be verified with further research. "We must remember that more than one factor is almost certainly involved in whether MS occurs," he said.