Solicitors for the family of a Cardiff woman murdered by her ex-partner say a judge has ruled a damages claim against two police forces can continue.
Joanna Michael, 25, dialled 999 twice during the attack by ex-boyfriend Cyron Williams in St Mellons in August 2009, but when officers arrived she was dead.
The Independent Police Complaints Commission (IPCC) found South Wales Police and Gwent Police had failed her.
A High Court judge sitting at Cardiff refused to strike out the claim.
The judge refused the police permission to appeal so they will have to go to the appeal court to challenge his ruling.
Ms Michael died when Williams broke into her home and found her with another man.
She suffered 72 separate wounds in the knife attack.
Williams was jailed for life in March 2010, with a recommendation that he serve 20 years, after admitting murder at Cardiff Crown Court.
The IPCC investigated after it emerged the mother-of-two had made two potentially life-saving 999 calls but police took 22 minutes to respond and did not arrive until after she had been fatally stabbed.
In its report, the IPCC concluded that Ms Michael "was failed by Gwent Police, South Wales Police and the 999 system itself".
At the High Court in Cardiff, Gwent Police and South Wales Police had asked Judge Milwyn Jarman, sitting as a Deputy High Court judge, to strike out the family's damages claim.
The family's solicitor, Hywel Davies, said the two forces had sought to stop the case at an early stage by relying on a rule of law that the police owed no duty of care in relation to damage caused by third parties and where the subject of the legal case was an attack on the way the police chose to "investigate and suppress crime".
He said that since that judgement, the police had in effect been immune from legal attack on the grounds that the public interest was served not by an individual right to compensation but by the police being able to investigate crime and protect the public free from the shadow of litigation.
He added: "This case doesn't just affect Joanna's family, it goes right to the heart of the police's obligation to protect the victims of domestic abuse."
Ms Michael's mother Angela said: "We have been and remain devastated by Joanna's death and are determined that the fatal errors are never repeated again.
"Two women every week are killed by their partners and things never seem to improve.
"We obviously know it was not the police who murdered Joanna but they had the power to save her but didn't because of simple carelessness and a failure to follow their own procedures."
A South Wales Police spokesman said: "We understand that this is an extremely sad case and once again extend our sympathies to the family of Joanna Michael.
"We acknowledge that proceedings are being advanced by Joanna Michael's family and it is inappropriate to comment further at this stage."
A Gwent Police spokesperson said: "This is a tragic case. However, it would be inappropriate to provide any further comment at this stage."