They believe it may be caused by the body's reaction to stress which effectively leads to the immune system becoming overloaded.

Researchers found that people with CFS produced unusually high levels of the hormone melatonin, which stimulates the immune system and plays an important role in promoting sleep.

Melatonin is vital for maintaining the body's correct diurnal rhythms - helping a person sleep at night and stay awake during the day. For this reason it is used to treat insomnia and jet-lag.

The researchers at Guy's and St Thomas's Hospitals, London, are convinced that the hormone is the key to CFS.

They believe their findings may lead to the much-misunderstood condition being re-classified as an immune system disorder.

Some doctors still believe CFS to be "all in the mind", although most now agree that to some degree it has a physical cause.

The symptoms of CFS include debilitating fatigue, neurological problems, general pain, gastro-intestinal problems and a variety of flu-like symptoms.

Severity differs widely

Chronic fatigue symptoms are similar to flu

The degree of severity can differ widely among patients, and will also vary over time for the same patient.

Severity can vary between getting unusually fatigued following stressful events, to being totally bedridden and completely disabled. The symptoms will tend to wax and wane over ime.

Dr Theodore Soutzos and Dr Ram Seth discovered the melatonin link after measuring daily production of the hormone in 44 CFS patients and 17 healthy volunteers.

They found that melatonin levels were higher in the CFS group as a whole. When patients with "active" symptoms - as opposed to those who were "improving" - were looked at in isolation, the pattern became "massively significant".

The findings were presented at the Royal College of Psychiatrists' annual meeting in Belfast.

Pathological problem

Dr Soutzos said: "What is interesting is that melatonin is an immuno-enhancer; it drives the immune system. If you deprive mice of melatonin, for instance, they start dying of cancer.

"One theory could be that some people drive their immune systems so hard that it becomes pathological. In certain individuals with an inherent vulnerability it could set off chronic fatigue syndrome."

This in turn could be stress-related, which would explain how psychological and physical factors could work together in CFS, said Dr Soutzos.

"People who drive themselves excessively, and are under stress artificially, put their immune systems under pressure," he said. "There is a negative feedback system which returns things to normal, but all you need is a slight imbalance and you get a vicious cycle."

A possible answer would be to treat the problem aggressively at the outset, before the vicious circle becomes established.