A US woman who was just an hour away from becoming the first female prisoner executed in Tennessee in more than 200 years has been granted a dramatic last-minute reprieve.

Christa Gail Pike, 50, had been scheduled to die by lethal injection at Nashville’s Riverbend Maximum Security Institution on Wednesday morning, local time, for the brutal 1995 murder of 19-year-old Colleen Slemmer.

But with witnesses already gathered at the prison and preparations for the execution under way, the US Court of Appeals for the Sixth Circuit intervened, issuing a temporary stay at around 9am – just an hour before Pike was due to be put to death.

In a 2-1 decision, the appeals court said the gravity and finality of the situation required more time to properly consider arguments raised by Pike’s legal team.

“The interests of justice and the finality of Pike’s impending execution compel the entry of a short stay of execution in order to properly analyze the parties’ fully briefed arguments and resolve the issues on the merits,” Circuit Judge Jane Stranch wrote.

The extraordinary intervention came after Pike had seemingly exhausted her remaining avenues for avoiding execution.

Tennessee Governor Bill Lee had rejected her request for clemency earlier in the week, while the US Supreme Court had also declined a request from her lawyers to halt the execution.

Pike was just 18 when Slemmer, a fellow student at a Job Corps training centre in Knoxville, was murdered in January 1995.

Prosecutors said Pike, her then-boyfriend Tadaryl Shipp and another teenager lured Slemmer to an isolated wooded area, where the 19-year-old was tortured and killed.

Pike was convicted of first-degree murder and sentenced to death in 1996. Shipp, who was 17 at the time of the killing, was sentenced to life in prison with the possibility of parole.

Pike has never denied her role in Slemmer’s murder and has expressed remorse for the crime.

“I was a mentally ill 18-year-old kid,” she said in a statement included in her clemency petition.

“It took me numerous years to even realize the gravity of what I’d done. Even more to accept how many lives I affected. I took the life of someone’s child, sister, friend.”

Her lawyers have argued that Pike’s age, mental health and history of severe childhood abuse were not adequately considered when she was sentenced to death.

The latest legal dispute centres on claims that evidence relating to childhood sexual abuse and rape was not properly weighed during earlier proceedings. The appeals court is now considering whether Pike’s latest filing can reopen aspects of her previous federal case.

Slemmer’s family, however, has long supported the execution.

Her mother, May Martinez, has spoken of the continuing pain of losing her daughter and had travelled to Tennessee ahead of Pike’s scheduled death.

“Not a day goes by, or a minute goes by that I do not think about Colleen,” she said in the days before the planned execution. “Holidays are the worst.”

The reprieve may yet prove temporary.

Tennessee authorities moved quickly to challenge the appeals court decision, asking the US Supreme Court to lift the stay and allow the execution to proceed.

“We will continue to advocate for the lawfully imposed sentence as the courts work through this technical issue,” the Tennessee Attorney General’s Office said.

If the current stay remains in place beyond the execution window, Tennessee’s Supreme Court would need to set a new execution date should the state ultimately be permitted to carry out Pike’s sentence.

For now, however, an execution that appeared certain to make history has been stopped at virtually the final moment – leaving Pike’s fate once again in the hands of the courts.