A Scottish nurse may have been wrongly convicted of the murders of four women, a BBC investigation suggests. Colin Norris was found guilty of poisoning five patients with insulin, one of whom survived, at two hospitals in Leeds in 2002. Norris was jailed for life in 2008 but new evidence is to be referred to the Criminal Cases Review Commission. New studies suggest naturally-occurring hypoglycaemia is much more common than the jury were led to believ Norris was convicted of the murder of four elderly women and the attempted murder of a fifth after a five-month trial in 2008. Despite no direct evidence linking Norris to any of these patients, he was on shift when they all had similar hypoglycaemic episodes - that is when the blood sugar drops to dangerously low levels. The prosecution argued that naturally-occurring hypoglycemia was so rare that a cluster of four or five cases must mean foul play. Norris was sentenced to at least 30 years in prison. But Prof Vincent Marks, one of the world\'s foremost experts on insulin poisoning, has carried out a forensic international review of all the new medical research. He believes there was insufficient evidence for insulin in four of the cases. Prof Marks said: \"I was surprised at how very common it is in this particular group of elderly, sick people \"In one very detailed survey, of thousands of patients, it was up to 10%. It\'s not that rare after all.\" Prof Marks added: \"These patients all had other risk factors which included emaciation, starvation, infection, cardiac failure, renal failure - they were all at very high risk of developing spontaneous hypoglycaemia. \"Looking at all the evidence, all I can say is I think Colin Norris\'s conviction is unsafe.\" A second expert, Dr Adel Ismail, believes that a crucial blood sample taken from the fifth patient suggesting insulin poisoning could also be wrong. Dr Ismail said: \"One additional sample taken a few hours earlier or later than the one, the single one, which is used, would have been immensely helpful, and, it wasn\'t done which is very unfortunate. \"The entire case was built on a foundation which is unsound.\" The BBC has uncovered evidence of other similar cases of hypoglycaemia which occurred in the hospital where Norris worked but while he was off duty. His lawyer, Jeremy Moore, believes there were serious flaws in the investigation and the convictions need to be quashed. He said: \"It seems that they trawled through hospital records looking for evidence of patients that might have died suspiciously but it seems they only cherry-picked those cases when Colin was on duty and ignored any others that might have occurred in the hospital.\" A spokesman for West Yorkshire Police said: \"Norris was arrested, prosecuted and on the basis of the evidence presented to the court he was convicted and sentenced.\" An application to hear a fresh appeal is to be sent to the Criminal Cases Review Commission.