Question marks have been raised over a nurse’s convictions of the murders of four elderly women, a BBC investigation has suggested.

Staff Nurse Colin Norris was handed four life sentences in 2008 by a judge at Newcastle Crown Court for murdering Ethel Hall, 86, of Calverley, Doris Ludlum, 80, of Pudsey, and Bridget Bourke, 88, and Irene Crookes, 79, both of Leeds, while he worked at Leeds General Infirmary and St James's Hospital in 2002.

A 20-year minimum sentence, to run concurrently, was also imposed for the attempted murder of 90-year-old Vera Wilby, of Rawdon, who survived a coma induced by an insulin injection.

Norris won the right to appeal against his convictions in 2009 but the jury’s findings were upheld later that year.

However, last night a BBC1 Scotland programme called Hospital Serial Killer: A Jury in the Dark was screened revealing new studies that suggest naturally-occurring hypoglycaemia is much more common that the jury were led to believe.

An application to hear a fresh appeal is now to be sent to the Criminal Cases Review Commission.

Norris had been working when all the pensioners had similar hypoglycaemic episodes and their blood sugar dropped to dangerously low levels.

The prosecution had argued naturally-occurring hypoglycaemia was so rare that a cluster of four or five cases must have meant foul play and Norris was sentenced to at least 30 years behind bars.

But one of the world’s foremost experts on insulin poisoning Professor Vincent Marks has carried out a forensic international review of all the new medical research and believes there was not enough evidence in four of the cases.

He said: “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 in unsafe.”

Another expert Dr Adel Ismail said in the programme he believed a crucial blood sample taken from a fifth patient suggesting insulin poisoning could also be wrong, saying: “The entire case was built on a foundation which is unsound.”

The BBC also uncovered evidence of other similar cases of hypoglycaemia in the hospital where Norris worked but while he was off duty.

His lawyer Jeremy Moore said the convictions needed to be quashed because of serious flaws. Mr Moore said: “It seems 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. His conviction was upheld at the Court of Appeal in December 2009.”

Leeds Teaching Hospitals Trust said it was a police prosecution and it would not be appropriate for it to comment.

Norris’s mother June Morrison, interviewed by BBC Radio Scotland, has insisted her son, who is “coping” in prison, is innocent of the murders and she has welcomed the new evidence.

Hospital Serial Killer can be watched on the BBC’s iPlayer.