Double murderer Hazel Stewart has failed in a legal bid to secure a reduced jail sentence for murdering her policeman husband and the wife of her ex-lover.

The 62-year-old mounted a fresh challenge based on claims she was under the coercive control of Colin Howell, who was also convicted of the murders.

The Court of Appeal in Belfast denied Stewart permission to introduce new medical evidence to support her case that she had been suffering from a mental illness.

Lady Chief Justice Dame Siobhan Keegan ruled her minimum 18-year jail term was neither wrong nor manifestly excessive.

“No injustice arises in refusing to reopen this long-concluded appeal on these facts.” Dame Siobhan said.

“This was a double murder of spouses in the cruellest of circumstances.”

Stewart is in prison for murdering her husband Trevor Buchanan, 32, and Howell’s 31-year-old wife Lesley Howell in 1991.

Howell is serving a 21-year sentence in HMP Maghaberry.

Brendan Kelly KC, acting on her behalf, had told the court new evidence had come to light from a psychiatrist who believed Stewart had been suffering from depression and PTSD at the time of the murders of Mr Buchanan and Mrs Howell.

He argued that at the time of Stewart’s sentencing in 2011 the issue of coercive control was not fully understood, in particular in the context of mental health.

He suggested had it been understood at the time of Stewart’s sentencing, the new evidence would have been admissible.

But Philip Henry KC, on behalf of the Public Prosecution Service, insisted there was no jurisdiction for the court to hear the case because Stewart previously abandoned her appeal.

He also repeatedly questioned the strength of the new diagnosis, obtained more than 30 years after the killings.

Six reports from other psychiatrists who examined Stewart at earlier stages raised no concerns about mental illness at the time of the killings.

Both victims were found in a fume-filled garage in Castlerock, County Londonderry.

Police originally believed the deaths were the result of a suicide pact after they discovered their partners were having an extra-marital affair.

Instead, they had been drugged and murdered before their bodies were arranged to make it look like they had taken their own lives.

It took nearly two decades for the truth to emerge when Howell suddenly confessed to both killings.

Howell also implicated his former lover in the plot and gave evidence against her at her trial.

During the trial, the court heard Howell had planned and carried out the killings and Stewart had facilitated them by drugging her husband, allowing Howell into her house and disposing of a hose pipe used in the murder.

In March 2011 Stewart was unanimously convicted of both killings by a jury at Coleraine Crown Court.

Speaking on Wednesday after the appeal, Lesley Howell’s family, referring to their mother by her maiden name of Lesley Clarke, said they were relieved that this stage of the legal process was over, but there is “no joy and no celebration”.

“While this decision offers some reassurance, we recognise that there’s no true end or closure.

“This is a life sentence, and we must continue to live with the pain caused by Hazel’s ongoing attempt to evade justice,” the statement added.

“Nothing will bring our mum back,” her children said.

They said their “beautiful mum’s memory will not be shadowed by this, and we will continue to love and honour the person she was”.