The 'hockey stick' that became emblematic of the threat posed by climate change exaggerated the rise in temperature because it was created using 'inappropriate' methods, according to the head of the Royal Statistical Society.
Professor David Hand said that the research – led by US scientist Michael Mann – would have shown less dramatic results if more reliable techniques had been used to analyse the data.
Prof Hand was among a group of experts charged with investigating the "climategate" email scandal that engulfed the University of East Anglia's Climatic Research Unit (CRU) last year.
Sceptics claimed that the hacked messages showed scientists were manipulating data to support a theory of man-made global warming.
However the review, led by Lord Oxburgh into the research carried out by the centre, found no evidence of ''deliberate scientific malpractice".
Lord Oxburgh said the scientists at the research unit arrived at their conclusions ''honestly and sensibly''.
Updated below with comments by Piers Corbyn
But the reviewers found that the scientists could have used better statistical methods in analysing some of their data, although it was unlikely to have made much difference to their results.
That was not the case with some previous climate change reports, where "inappropriate methods" had exaggerated the global warming phenomenon.
Prof Hand singled out a 1998 paper by Prof Mann of Pennsylvania State University, a constant target for climate change sceptics, as an example of this.
He said the graph, that showed global temperature records going back 1,000 years, was exaggerated - although any reproduction using improved techniques is likely to also show a sharp rise in global warming. He agreed the graph would be more like a field hockey stick than the ice hockey blade it was originally compared to.
"The particular technique they used exaggerated the size of the blade at the end of the hockey stick. Had they used an appropriate technique the size of the blade of the hockey stick would have been smaller," he said. "The change in temperature is not as great over the 20th century compared to the past as suggested by the Mann paper."
The "hockey stick" was used to warn the world of the threat of global warming by numerous individuals and organisations, including Al Gore in his oscar-winning film an Inconvenient Truth and UN body the Intergovernmental Panel on Climate Change.
But sceptics immediately pounced on the graph and whole books have been written questioning its key finding.
The graph used data from hundreds of studies of past temperatures using tree rings, lake sediment, and glacier ice cores and then merged these with more reliable recent temperature records.
Prof Hand said many of the reproductions of the graph do not make clear when these different sets of data are used.
"It is only misleading in the sense they merged two different things," he said.
Prof Hand praised the blogger Steve McIntyre of Climate Audit for uncovering the fact that inappropriate methods were used which could produce misleading results.
"The Mann 1998 hockey stick paper used a particular technique that exaggerated the hockey stick effect," he said.
Prof Mann, who is Professor of Earth System Science at the Pennsylvania State University, said the statistics used in his graph were correct.
"I would note that our '98 article was reviewed by the US National Academy of Sciences, the highest scientific authority in the United States, and given a clean bill of health," he said. "In fact, the statistician on the panel, Peter Bloomfield, a member of the Royal Statistical Society, came to the opposite conclusion of Prof Hand."