Six Lancashire police employees are facing misconduct proceedings after a "shocking catalogue of errors" left a vulnerable woman at the mercy of a violent attacker.
The victim was stabbed and had boiling water poured over her despite asking the police for protection.
She had spoken to them several times on the day of the ordeal but was "left alone and scared".
She sustained serious injuries despite an urgent request from a police officer earlier in the day that her tormentor should be arrested.
A manager at the nursery attended by the woman's children and a social worker also contacted the police.
The police complaints authority has published a scathing report saying that it was "a tragedy that the woman had to go through such a terrible experience" for the Lancashire force to learn lessons.
The Independent Police Complaints Commission (IPCC) concluded that "error after error" had resulted in "a failure to recognise the increasing risk" to the woman, who has not been named.
Commissioner Ms Naseem Malik said: "This was a vulnerable woman who was terrified that an abusive ex-partner was intent on causing her harm. That turned into terrifying reality.
"She turned to Lancashire Constabulary for help but a shocking catalogue of errors left her alone and scared.
"Opportunities were clearly missed to deal with her fears sooner and give her the protection she needed."
The IPCC revealed that the woman was a "repeat victim" of domestic violence and should have been treated as a priority.
It has recommended that police call handling be improved.
Lancashire police have apologised to the victim, accepting that their response was inadequate.
Assistant Chief Constable Andy Cooke said: "We have already made a number of improvements following this incident such as additional training for communications room staff including improving awareness around recognising the vulnerability of callers and the threat posed to them."
Five communications operators and a police sergeant are to face disciplinary proceedings
The woman's ex-partner, Craig Clarke, was jailed for six years after admitting grievous bodily harm with intent.
At the time of the attack he was on bail in connection with another assault.