A CONVICTED killer who had served 25 years behind bars died after taking an insulin overdose in one of Britain's top security prisons, an inquest has heard.

Diabetic Philip Whiteman died in hospital three weeks after he was found unresponsive in his cell at Durham's Frankland Prison last November.

The 46-year-old was serving life after he used his dressing gown belt to strangle a fellow patient at the high security Ashworth Hospital, Liverpool, in 1990.

An inquest before a jury at Crook, County Durham, heard he had admitted manslaughter on the grounds of diminished responsibility.

Whiteman, who had a troubled childhood in Newcastle and was diagnosed as having a personality disorder, was treated at secure hospitals such as Broadmoor and Rampton.

In January 2013 he was sent to Durham Prison after he absconded from the medium-secure St Nicholas's Hospital in Newcastle, and he was transferred to Frankland in February last year.

Pathologist Dr Mark Egan said Whiteman died as a result of hypoglycemic brain injury, caused by an overdose of insulin.

Dr Steve Ramplin, a consultant forensic psychiatrist, said Whiteman felt a "strong sense of injustice" that he had remained behind bars for so long.

Detective Sergeant Anna Phillips told Durham Coroner Andrew Tweddle that Whiteman had written a note which was found on the bed in his cell and there was no evidence of any third party involvement in his death.

"Mr Whiteman was in a single cell, and had been found by prison staff, apparently having taken an overdose of insulin," she told the jury.

The inquest, which was scheduled to last four days, continues.