Kenneth Smith, 58, was sentenced to death after being convicted, along with a co-defendant, for his role in the 1988 murder of Elizabeth Sennett.

Mrs Sennett’s husband, a preacher, had offered the two men $1,000 each to kill her. He later took his own life when he was suspected of involvement.

Although the jury in his trial recommended life imprisonment without parole by 11-1, the sentencing judge overrode this recommendation and imposed the death penalty.

Mr Smith was initially due to be executed by lethal injection in November 2022, but the execution was called off after an hour as prison staff couldn’t locate a suitable vein before the death warrant expired.

The botched execution was described by an appeal judge as “horrifying” and “severely painful”, and by the Supreme Court of the United Sates as “torturous” and involving “severe pain and suffering”.

He is now set to become the first prisoner in the world to be executed through the ‘nitrogen hypoxia’ method, despite protests from human rights experts and campaigners.

UN experts last week urged Alabama to reconsider the use of the untested method on the basis that it could “result in a painful and humiliating death”.

A spiritual adviser who has agreed to accompany Mr Smith during the execution has been told the method could also endanger others in the room, he wrote in The Appeal.

Amnesty International has called on Alabama’s Republican Governor Kay Ivey to use her clemency power to stop the execution.