A US judge has granted a stay of execution to Lisa Montgomery - just hours before the only woman on America's federal death row was due to be given a lethal injection.

Judge James Hanlon said a competency hearing should be held first.

Montgomery's lawyers had argued she was mentally incompetent to be executed, saying she was born brain-damaged.

Lisa Montgomery strangled a pregnant woman in Missouri before cutting out and kidnapping the baby in 2004.

Montgomery, now 52, would have been the first female federal inmate to be put to death in almost 70 years had her execution in Terre Haute, Indiana, gone ahead as scheduled on 12 January.

What did the judge rule?
In Monday's ruling, Judge Hanlon of the US District Court for the Southern District of Indiana wrote: "Ms Montgomery's motion to stay execution is granted to allow the court to conduct a hearing to determine Ms Montgomery's competence to be executed.

"Ms. Montgomery's current mental state is so divorced from reality that she cannot rationally understand the government's rationale for her execution.

"The court will set a time and date for the hearing in a separate order in due course," Judge Hanlon wrote.

Montgomery's attorneys welcomed the ruling.

"The court was right to put a stop to Lisa Montgomery's execution," attorney Kelley Henry said in a statement.

"As the court found, Mrs Montgomery 'made a strong showing' of her current incompetence to be executed.

"Mrs Montgomery has brain damage and severe mental illness that was exacerbated by the lifetime of sexual torture she suffered at the hands of caretakers," the attorney added.

Prosecutors have filed a notice to appeal the judge's ruling, CNN reports.

Can Montgomery escape death penalty?
It is the second time that Montgomery's execution has been postponed.

Her execution date was originally set for last December - but a stay was put in place after her attorneys contracted Covid-19.

The last woman to be executed by the US government was Bonnie Heady, who died in a gas chamber in Missouri in 1953, according to the Death Penalty Information Center.

Federal executions had been on pause for 17 years before President Donald Trump ordered them to resume earlier last year.

There are suggestions that Montgomery now may even escape the death penalty entirely, as President-elect Joe Biden - who is due to take office on 20 January - has said he will seek to end federal executions.