British scientists, however, have discovered the first firm evidence that the illness — also known as chronic fatigue syndrome (CFS) — leads to clear biological changes in the blood, offering the strongest indication yet that its trigger is physical. The findings, from a team at Imperial College, London, promise to lead to the first reliable blood test for the condition, also known as myalgic encephalomyelitis or ME.

It afflicts at least 150,000 people in Britain with crippling exhaustion and concentration problems. Well-known sufferers include Lord Puttnam, the director, Dame Kelly Holmes, the athlete, Shirley Conran, the author, and Emily Wilcox, the daughter of Esther Rantzen, the television presenter.

The work could also bring new approaches to treating the syndrome and, just as importantly for many sufferers, end the long-standing medical controversy over its causes.

CFS patients and their support groups have long been angered by the insistence of some doctors that their condition is primarily psychological, or even that it is an invented illness used as a malingerer’s excuse. Many are adamant that it must be triggered by an external event, such as exposure to a virus or environmental toxins.

Though few doctors now think the condition is wholly invented, advocates of the psychological explanation think it more likely to be caused by chemical imbalances in the brain, in similar fashion to mental illnesses such as depression or schizophrenia.

While the new research is not incompatible with a psychological origin, it suggests that a physical cause, such as viral infection, is more likely.

The team led by Jonathan Kerr found that genes in the white blood cells are switched on and off in abnormal fashion in patients with the illness, and the nature of this changing pattern points towards viruses as the probable culprits.

Though the study was small, including only 25 patients, it is the first to find a consistent pattern of change. If its outcome is confirmed by larger investigations, it could settle the debate about the condition.

In the research, Dr Kerr compared the way genes are activated in immune system cells in 25 patients and 25 healthy control subjects. The pattern in which genes are switched on and off — known as gene expression — can often be critical to biological processes in the body. The analysis of 9,522 genes identified 35 that appeared to show differences, and a more precise examination of these revealed significant and genuine changes in the way that 16 of the genes were expressed in the CFS group.

All but one of these sixteen genes were up to four times more active in CFS patients than in the controls, while one was less active. The results will be published in the peer- reviewed Journal of Clinical Pathology next month.

Dr Kerr, whose team is moving to St George’s, University of London, is now repeating the experiment in a larger group of 1,000 patients and controls. New Scientist magazine reports today that the bigger study has so far reached similar conclusions.

He said the results support a theory that the condition is often triggered by viruses such as Epstein-Barr, Q fever, enteroviruses and parvovirus B19, which causes lasting changes in gene expression that lead to chronic fatigue. “CFS often begins with a flu-like illness which never goes away,” Dr Kerr said.

Many of the genes identified as different affect the functioning of the mitochondria — the tiny structures that generate energy within cells.

“The involvement of such genes does seem to fit with the fact that these patients lack energy and suffer from fatigue,” Dr Kerr said.

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