Ruth Ellis, the last woman to be executed in Britain, was suffering from "battered woman syndrome" when she shot her lover dead, three appeal judges were told on Tuesday.
Ruth Ellis shot David Blakely six times outside a London pub
However, the jury at her 1955 Old Bailey trial was not allowed to consider her defence of provocation, said Michael Mansfield, QC.
He said the trial judge, Sir Cecil Havers, fell into "substantial error" in withdrawing that defence from the jury, which took just 15 minutes to convict Ellis of murder.
That mistake by the judge led to a miscarriage of justice which had lasted for 48 years, Mr Mansfield told the Court of Appeal in London.
Ellis, a 28-year-old nightclub hostess, was hanged in 1955 for killing racing driver David Blakely as he emerged from the Magdala pub in South Hill Park, Hampstead, north London.
Ellis's grandchildren and her 82-year-old sister, Muriel Jakubait, of Woking, Surrey, are leading the family's fight to establish she should never have gone to the gallows.
Lord Justice Kay, Mr Justice Silber and Mr Justice Leveson were told that 10 days before the killing Ellis had suffered a miscarriage after Blakely, the baby's father, punched her in the stomach.
Lawyers also pointed to the role played by another boyfriend, who was said to have given her a gun and taught her to use it before driving her to the murder scene.
Mr Mansfield said the trial judge, as well as the prosecuting and defence counsels, were "labouring under a misconception or misunderstanding of the law" in relation to the disallowed provocation defence.
But David Perry, for the Crown, argued that the law of provocation did not arise in Ellis's case because there was no final provocative act and no evidence of a
sudden and temporary loss of control.
"Even if the accused responded to conduct then recognised as a form of provocation, the plea could be dismissed if the response was wholly
disproportionate," he said.
"A reasonable woman would not have been provoked on the basis of the alleged provocative act."
This was not a crime of passion, said Mr Perry, but a "calm, deliberate, premeditated killing".
Mr Mansfield said he would be seeking to call fresh evidence from a medical expert who believes Ellis was suffering from what was now characterised as "battered woman syndrome".
The hearing has been adjourned until Wednesday