Fresh analysis of a controversial study, which recommended exercise and psychological therapy for people with chronic fatigue syndrome, suggests their impact is more modest than first thought.
The PACE trial found the treatments to be "moderately effective", leading to recovery in a fifth of patients.
But this new analysis finds "no long-term benefits at all".
The authors of the original trial in 2007 said they stood by their findings.
That randomised trial was designed to examine the effectiveness of graded exercise therapy (GET) and cognitive behavioural therapy (CBT) for chronic fatigue syndrome (CFS), also known as myalgic encephalomyelitis, or ME.
Its findings were positive, but patient groups like the ME Association have always been critical of the way the trial was designed and the way the results were reported.
There has also been controversy over the release of data from the trial, with some arguing it should be made available to all researchers for further analysis.
The PACE trial reported that 59% of patients who received CBT and 61% who had exercise therapy had improved overall, compared with 45% in a control group.
When the results were re-examined, after data was obtained under a Freedom of Information request, researchers found that just 20% of CBT patients and 21% of GET patients improved, along with 10% of control patients.
Figures for those who recovered were originally reported as 22% for patients in each of the CBT and GET groups, but this reduced to 8% in the latest re-analysis.
Writing in the journal BMC Psychology, lead author Dr Carolyn Wilshire, from the University of Wellington in New Zealand, said the PACE trial moved the goalposts by changing the way treatment success was measured after the trial had begun.
She added: "Until there is positive evidence to suggest otherwise, the conclusion we must draw is that PACE's treatment effects are not sustained over the long term, not even on self-report measures.
"CBT and GET have no long-term benefits at all. Patients do just as well with good basic medical care."
Modestly effective treatment
The ME Association, which part-funded the new study, said it was no surprise that "impressive claims for recovery following CBT and GET are not statistically reliable".
Dr Jon Stone, consultant neurologist at the Western General Hospital in Edinburgh, said better treatments for chronic fatigue syndrome were needed, or more effective forms of rehabilitation.
"Until we have these, the question is whether it is better to offer a modestly effective treatment supported by data from many other trials, with a realistic discussion of its pros and cons, than none at all."
The three authors of the original PACE trial – Prof Michael Sharpe, from the University of Oxford, and Prof Trudie Chalder and Dr Kimberley Goldsmith, from King's College London, said the new analysis had used only part of the data from the trial.
They also said many other trials and meta-analyses had replicated the findings of the PACE trial.
"In conclusion, we find little of substance in this critique and stand by our original reports."
NICE is currently updating its guidance on the diagnosis and management of chronic fatigue syndrome.