A psychiatric patient has been jailed for life with a minimum term of 37 years for killing a grandmother in a street knife attack.
But the police watchdog said officers failed to carry out a check on Nicola Edgington on the day of the murder.
Nicola Edgington, 32, walked out of a mental health unit to buy a knife at a supermarket before she attacked two women.
Edgington, who had previously killed her mother, murdered a woman in the street following a series of police blunders, the watchdog said.
She had made five 999 calls on the day of the attack asking to be sectioned under the Mental Health Act because she believed herself to be a danger.
But hours later she took a knife from a butcher's shop and almost decapitated grandmother Sally Hodkin, 58, after she attempted to murder Kerry Clark, 22, at a bus stop.
The 2011 killing, in Bexleyheath, south-east London, came after Edgington was released back into the community in 2009.
An investigation by the Independent Police Complaints Commission (IPCC) also found Met staff failed to carry out a police national computer (PNC) check on Edgington when she made contact with officers on October 10 2011, shortly before the murder.
This would have alerted them to her previous conviction for the manslaughter of her mother in Sussex, the IPCC said.
Hours before she killed, Edgington sought help at a local hospital from where she called 999.
But after delays in admitting her, she walked out.
The IPCC found officers only contacted the hospital following her fourth phone call and said Edgington's second 999 call from the A&E department was downgraded because she was considered to be in a place of safety.
An officer was not dispatched at this stage despite Edgington's claim she could be very dangerous, it found.
The life sentence for Edgington came on the day that the Independent Police Complaints Commission found failings in the murder case:
- Local police in Greenwich were not notified that Edgington was living in the area.
- Police and police staff did not carry out a police national computer check on Edgington which would have alerted them to her previous conviction for manslaughter.
- Officers missed an opportunity to use their powers under the Mental Health Act when Edgington tried to leave an A&E department shortly after she arrived with police.
- Edgington's second 999 call from an A&E department was downgraded because she was considered to be in a place of safety and an officer was not asked to return despite Edgington saying she could be very dangerous.
- The police only contacted the hospital after a fourth call had been received.
IPCC commissioner Sarah Green said it was of "great concern" that police did not carry out a PNC check.