The potential breakthrough offers fresh hope for ME patients. There is currently no cure for the condition, which is also known as chronic fatigue syndrome (CFS).
US researchers led by Dr Judy Mikovitis, from the Whittemore Peterson Institute at the University of Nevada, studied patients' white blood cells.
Samples were taken from 101 patients who had ME and compared with samples from 218 patients who did not have the condition.
All the patients with ME had severe disability, prolonged disabling fatigue and immune system abnormalities.
The researchers found that a retrovirus called XMRV was present in 67 per cent of the patients with ME. The virus was only present in 3.7 per cent of people without the condition.
They sequenced the genetic code of the virus strain present in the ME patients. The XMRV strain was found to be similar to that linked to prostate cancer.
The researchers also discovered that it could be transmitted to prostate cancer cells in the laboratory.
Will the discovery herald new treatments for ME?
Dr Mikovitis said the discovery could be 'a major step in the discovery of vital treatment options for millions of patients'.
The researchers stressed, however, that at present the findings only show an association between ME and the XMRV virus and do not prove that the virus causes ME.
Dr Robert Silverman, one of Dr Mikovitis's co-workers added: 'If cause-and-effect is established, there would be new opportunity for prevention and treatment of these diseases.'
What do other researchers say?
Tony Britton, of the ME Association, said the research was interesting.
'Many people with ME/CFS say their illness started after a viral infection, and a number of enteroviruses and herpes viruses have also been implicated in the past,' he commented.
'This is fascinating work, but it doesn't conclusively prove a link between the XMRV virus and CFS or ME. It is an immensely complex illness, with many possible causes. There are up to 240,000 patients in the UK desperate to get better,' he added.
- The XMRV virus has been found in 67 per cent of ME cases.
- The virus has been linked to cases of prostate cancer.
- Research will need to show the virus causes ME before new treatments can be developed.