A SOUTH WALES family has lost its bid in Supreme Court for the right to sue police for negligence over the death of a young mother killed by her ex-boyfriend in ''a mad fit of jealous rage''.

Joanna Michael, 25, from St Mellons, dialled 999 twice, but ''individual and systemic failures'' by both Gwent and South Wales police forces meant the emergency services arrived too late to save her life, the highest court in the land was told last July.

But the court backed the family's right to take their case to under the European Convention on Human Rights which the family say were were breached by a police failure to protect Joanna's life The Michael family asked the the Supreme Court to overturn an appeal court ruling in 2012 that the police have an ''immunity'' from being sued for negligence under common law for the actions of officers during ''the investigation or suppression of crime''.

But the Supreme Court justices decided by a 5-2 majority today that the judges were right and dismissed the family's appeal.

The seven justices unanimously dismissed a cross-appeal by the chief constables of South Wales and Gwent Police.

They failed to block the appeal court judges' linked ruling that the family should be allowed to go ahead with a claim that their Article 2 rights under the European Convention on Human Rights were breached by a police failure to protect Joanna's life Nicholas Bowen QC, appearing for Ms Michael's family, told the Supreme Court the case was ''desperately important'', particularly with regard to cases of domestic violence.

He told the court panel - Lord Neuberger, Lady Hale, Lord Mance, Lord Kerr, Lord Reed, Lord Toulson, Lord Hodge: ''There is a need for a heightened accountability of the police in the light of recent scandals and investigations which have had a very serious detrimental affect on public and political confidence in police services.''

He described how Ms Michael was brutally killed in August 2009 by ex-boyfriend Cyron Williams, 19.

Williams broke into her home ''in a mad fit of jealous rage after he discovered she was in a new relationship some weeks after they had finished seeing each other''.

There was a history of domestic abuse, and Williams is now serving life with a 20-year tariff, which means that he will remain in prison at least to 2030.

Ms Michael made her first 999 call on a mobile phone to the police at 2.29am on August 5 2009 and told the Gwent Police operator that Williams had come to the house and found her with someone else.

He had bitten her ear hard and taken the other man away in his car - saying he would return to kill her.

Mr Bowen said the ''urgency was absolutely plain'' and an immediate response could have meant police reaching her in five minutes. But the call went through Gwent - ''the wrong police force'' - and not South Wales, as it should have done.

The Gwent operator told the mother of two to ''stay put'' in the house and keep the phone free as South Wales Police would want to call her back, said Mr Bowen.

According to the appeal court judgment, the Gwent operator spoke to her South Wales Police counterpart and said Williams had threatened to return to ''hit'' Ms Michael but did not refer to ''the threat to kill''.

The call should have continued to be graded as requiring an immediate response, but was instead graded at the next level down.

A further 999 call was received by Gwent Police from Ms Michael at 2.43am and she could be heard screaming before the line went dead.

Police officers arrived at 2.51am. Ms Michael was found to have been stabbed by Williams 72 times.

Mr Bowen told the court that the ''factual picture'' revealed there were ''crucial legal duties owed to Ms Michael's estate by the police''.

He argued that officers had failed to arrive in time and possibly save her life because of unacceptable delays. These were caused by individual and systemic errors of the police which justified them having to face damages claims for negligence.

Mr Bowen said the police were claiming immunity from being sued, largely relying on the 1989 case of Hill v Chief Constable of West Yorkshire - the ''Yorkshire Ripper'' case.

The Independent Police Complaints Commission (IPCC) has already ruled that Ms Michael was failed by both South Wales and Gwent police.

Gwent and South Wales Police released this statement following the judgement this morning: “Today’s judgement resolves an important point of law which had implications beyond this case. An element of this claim still remains subject to legal proceedings and it would be inappropriate to comment further at this stage. However our thoughts, as ever, remain with Joanna’s family and all affected by the tragic impact of this case.

"South Wales Police and Gwent Police take domestic violence very seriously and both forces have worked hard to improve the way we respond to incidents of domestic abuse."