By Michelle Roberts
Health reporter, BBC News
The blood samples were free of XMRV
UK scientists say they can find no proof that a particular virus is the cause of chronic fatigue syndrome (CFS) or ME, contrary to recent claims.
The Imperial College London team say they want to share the findings as some patients are pinning their hopes on drugs to fight the virus called XMRV.
They analysed blood samples from 186 patients with CFS and found none had the virus, PLoS One journal reports.
Experts said the latest findings would be a bitter disappointment to many.
They said more trials were under way and when these report in coming months, scientists will be able to draw more firm conclusions.
Work in the US, published in Science, had found the retrovirus in 68 of 101 CFS patients.
The UK team say the conflict between the two studies might be down to differences between the patients enrolled or the way the research was conducted.
Or there might be different geographical types or strains of XMRV.
Regardless, they say potent antiretroviral drugs should not be used to treat CFS because there is not enough evidence that this is necessary or helpful.
The drugs may do more harm than good, they say.
Professor Myra McClure, one of the Imperial College London investigators, said: "We are confident that our results show there is no link between XMRV and chronic fatigue syndrome, at least in the UK."
She said they had used extremely sensitive DNA testing methods, called polymerase chain reaction, to look for the virus.
"If it had been there, we would have found it."
Co-author Professor Simon Wessely said the findings did not invalidate all previous research, some of which has shown that CFS can be triggered by other infectious agents, such as Epstein Barr Virus.
Causes chronic fatigue and muscle pain
Impairs immune system
Does not improve with sleep
Affects more women than men
Some have doubted it is a genuine physical illness - but the medical profession is now convinced
The charity Action for ME said it was disappointing to hear about these findings, but said no single small-scale study could be conclusive.
Dr Charles Shepherd, of The ME Association, said it was important to remain open-minded.
"We need to be extremely cautious until we know more. There has been enormous interest in this from patients.
"Some have been led into believing the cause and a test has been discovered and that treatment is just round the corner and that is not the case.
"Over the next few weeks and months we will have more results and then we can come to a conclusion.
"If it turns out that XMRV is important, we will have to start looking at whether it is worthwhile testing for it and treating it."
The team behind the initial US study said the latest work did not use the same "rigorous" techniques they had used and therefore the results were not comparable.
Dr Judy Mikovits from the Whittemore Peterson Institute said: "Little is known about the prevalence of XMRV worldwide, much less the incidence of XMRV in ME/CFS or prostate cancer.
She added more study was needed and they were collaborating with international research teams to unpick the questions around the virus.
In a statement, the charity Invest in ME said the original Science study was of the "highest quality", and that much more work was required before any firm conclusions could be drawn.