The murder convictions of a nurse dubbed the “angel of death” have been referred to the Court of Appeal.

Colin Norris, 44, was found guilty in 2008 by a majority of murdering four women and attempting to murder another by injecting them with insulin.

The four women who died – Ethel Hall, Bridget Bourke, Doris Ludlam and Irene Crookes – plus the fifth woman were all elderly inpatients on orthopaedic wards in Leeds where he worked as a nurse.

After reviewing the case, the Criminal Cases Review Commission (CCRC) has decided to refer all five convictions to the Court of Appeal.

It said: “The CCRC considered new expert evidence presented by Mr Norris’s representatives and instructed its own expert to provide a number of reports.

“This new expert evidence explored recent developments in a complex area where scientific understanding is still developing.

“As a result of the new expert evidence, the CCRC has concluded that there is a real possibility that the Court of Appeal will decide that Mr Norris’s conviction for the murder/attempted murder of one or more of the four patients is unsafe.”

Norris – who was born in Glasgow – was found guilty of four counts of murder and a single count of attempted murder after a five-month trial at Newcastle Crown Court.

He was sentenced to life imprisonment with a minimum term of 30 years.

The case against him was wholly circumstantial and heavily reliant on expert opinion evidence on a number of complex medical and scientific issues.

Experts have now agreed that the hypoglycaemia in four of the women – other than Mrs Hall – may be accounted for by natural causes, according to the CCRC.

Mrs Hall developed severe hypoglycaemia while in hospital and died on December 11 2002 and the cause of her murder is not being questioned.

However, the CCRC considers this conviction depends on support from the other four cases and the prosecution’s assertion that no-one other than Norris could have been responsible.

The CCRC said new evidence also highlighted several other relevant developments in the understanding of hypoglycaemia, including its prevalence in the elderly and frail.

This has cast further doubt on the expert opinion relied upon by the prosecution at trial.

The statement added: “The CCRC considers that this conviction depends upon support from the other four cases and the prosecution’s assertion that no-one other than Mr Norris could have been responsible.

“In light of the new expert evidence, the CCRC is satisfied that this assertion is now less secure and that, as a result, there is a real possibility that the Court of Appeal will quash this conviction too.”