During a three-year trial at Stanford University, a small group of ME patients who had shown symptoms related to the herpes virus were given the drug valganciclovir, an anti-viral drug that is often used to treat herpes-related diseases. Of the 25 patients treated, 21 responded with significant improvement. Those who responded to the drug had developed ME after an initial flu-like illness, while the non-responders had suffered no initial flu.

"This could be a solution for a subset of patients, but that subset could be quite large," said Kristin Loomis, executive director of the HHV-6 Foundation. "These viruses have been suspected in CFS for decades, but researchers couldn’t prove it because they are so difficult to detect in the blood."

ME has been classified by the World Health Organization as a neurological disorder that often begins with routine flu-like symptoms, but can result in years of chronic, painful fatigue. In January 2002, Liam Donaldson, the Government’s chief medical officer, declared ME a genuine chronic illness that may cause profound, prolonged illness.

The majority of ME sufferers range from young people in their teens to adults in their early thirties and forties. "Many people are still unaware that ME also affects children and young adults," said Wendy Holloway, a representative at AYME, a UK charity for young people with ME.

She said there were an estimated 30,000 children and young people in the UK suffering from the disorder, many of whom might not be able to adequately convey their symptoms to adults. "This can lead to delays in diagnosis, problems with education, and social isolation for the youngsters involved."

Stanford University professors, Jose Montoya and Andreas Kogelnik, who are both medical doctors, have received a grant for over $1 million (more than £50,000 pounds) from Roche Pharmaceutical, which manufactures the drug under the brand name Valcyte, to begin a new study immediately.

"What is desperately needed is the completion of a randomised, double-blind, placebo-controlled clinical trial that we are about to embark on," Montoya said.

The trial has been welcomed by the UK charity, the ME Association, whose medical advisor, Dr Charles Shepherd said: "I am very pleased to see that they are now going on to do a proper placebo controlled trial for this sub-group. The main hope is that we have a group of doctors taking these research findings seriously and applying them to an actual form of medical care. For many people ME is a long-term condition, and up this point we still do not have a treatment for the underlying disease process.

"Even if we see positive results from the new study, we should not expect to see the drug on the market immediately. Doctors will want to see positive results from at least two or three other independent groups, and we would have to be absolutely sure about it’s safety.

"In the best case scenario, I imagine it would be somewhere from 18 months to two years before the drug would be available to ME patients in the UK."