The family of a woman who was murdered by a former mental health patient are claiming damages against an NHS Trust.
Sally Hodkin was stabbed by Nicola Edgington, 32 in south east London with such force she was virtually decapitated.
Edgington, who was given a life sentence, was at Oxleas House when she walked out of the mental health unit and attacked the 58-year-old.
Mrs Hodkin's family claim NHS Oxley Foundation was negligent.
Edgington, who had a previous manslaughter conviction for killing her mother, was told she must serve a minimum 37 years in jail.
During the trial, the Old Bailey heard that Edgington from Greenwich was taken by police to Queen Elizabeth Hospital in October 2011 after staff at a cab firm became concerned by her behaviour.
While waiting to be admitted, she rang 999 five times, warning that she was dangerous and would harm somebody if she was not sectioned.
She was later taken to the mental health unit at Oxleas House, run by Oxleas NHS Foundation Trust, but walked out several hours later and attacked Mrs Hodkin.
The family claim the NHS Trust negligently failed to realise Edgington had a previous conviction for manslaughter and failed to detain and treat her as a danger to the public, which directly resulted in her death.
Following the murder conviction the judge also ordered Edgington to serve a minimum sentence of 20 years, which will run concurrently, for the attempted murder of Kerry Clark, 22, who she tried to stab shortly before Mrs Hodkin was killed.
Solicitor Daniel Rubinstein, acting for the family and submitting the damages to the High Court, said: "We allege that the hospital totally failed in its duties to protect those in the locality and should be held to account for this."
The NHS trust has refused to comment on the action.
Following the trial in February an Independent Police Complains Commission investigation found that no one had breached the code of conduct by not running a background check on Edgington.
A Police National Computer check would have revealed the previous conviction for manslaughter, but the IPCC concluded that a background check would not have prevented the actions Edgington took.
In 2005 Edgington stabbed her mother to death at her home in East Sussex, and pleaded guilty to manslaughter due to diminished responsibility.