Joanna Michael family in Supreme Court negligence fight

The family of a woman who made 999 calls for help before she was stabbed to death by her boyfriend wants the Supreme Court to let them sue two police forces for negligence.

Joanna Michael, 25, from St Mellons, Cardiff, rang 999 twice before Cyron Williams killed her in August 2009.

She was heard screaming during the second call before the line went dead.

The Independent Police Complaints Commission (IPCC) ruled that she was failed by South Wales and Gwent Police.

In 2011, Ms Michael's family won the right to try to bring a claim for damages against both police forces. It was thrown out by the Court of Appeal in 2012 because police officers have immunity from negligence claims. The family have now taken their case to the Supreme Court.

On the night of the killing, 19-year-old Williams broke into Ms Michael's home and found her there with another man.

Nicholas Bowen QC, for Ms Michael's family, told the Supreme Court Williams broke into Ms Michael's 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".

She made her first 999 call on a mobile phone to the police at 02:29 BST on 5 August, 2009 and told the Gwent Police operator Williams had come to the house.

Boyfriend Cyron Williams admitted killing Ms Michael

He had bitten her ear hard and taken the other man away in his car, saying he would return to kill her. Her young children were also in the house.

Mr Bowen said the "urgency was absolutely plain" and an immediate response could have meant police reaching her in five minutes.

However, the call had gone through Gwent - "the wrong police force" - and not South Wales.

The Gwent operator told the mother-of-two to "stay put" and keep the phone free as South Wales Police would want to call her back, the court heard.

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 02:43 BST and she could be heard screaming before the line went dead.

Officers arrived at 02:51 BST but in the 22 minutes it took officers to respond to her first call she had been stabbed by Williams 72 times.

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

Mr Bowen told the court officers had failed to arrive in time and possibly save her life because of unacceptable delays caused by individual and systemic errors which justified the police having to face damages claims for negligence.

Williams was jailed for life in March 2010 with a recommendation that he serve 20 years, after admitting murder at Cardiff Crown Court.

Mr Bowen said the case was "desperately important".

He told seven Supreme Court justices: "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 said the legal action was "not just about money" but also a "determination to find out really and truly what happened".

Ms Michael's call should have been classed as an immediate response, the court heard