The US Supreme Court will not block Alabama from executing Kenneth Eugene Smith with nitrogen gas, a method never used before for capital punishment.
Death row inmate Smith had asked the court to intervene saying the execution was a cruel and unusual punishment.
The execution, in which nitrogen would be pumped into his body through a mask, is planned for Thursday.
Smith, 58, also made a separate legal challenge with a lower court – but that has now also rejected his request.
Alabama already tried to execute the convicted murderer by lethal injection two years ago.
But the attempt failed because executioners were unable to raise a vein before the state’s death warrant expired at midnight.
Alabama is now trying again, this time by using nitrogen gas.
It would make Smith the first person ever to be executed by this method, according to the Death Penalty Information Center, which researches capital punishment in the US and around the world.
Smith was one of two men convicted in 1989 of murdering a preacher’s wife, 45-year-old Elizabeth Sennett. She was stabbed and beaten to death in a $1,000 (£790) killing-for-hire.
At his trial Smith admitted to being present when the victim was killed, but says he did not take part in the attack.
The UN’s High Commissioner for Human Rights has said gassing Smith could amount to torture or other cruel, inhuman or degrading treatment, and called for a halt.
Smith’s lawyers lodged a legal challenge with the Supreme Court, arguing that putting convicts through multiple execution attempts violates the Eighth Amendment of the US Constitution, which protects against “cruel and unusual” punishment.
His lawyers also said the nitrogen gas method was “recently released and untested”, leaves him at risk of choking on his own vomit, and is “a novel method of execution that has never been attempted by any state or the federal government”.
Smith – who has been on death row for more than three decades – spoke to the BBC via written answers earlier this week, where he said the wait felt like “torture”.
Lawyers for the state, said in a court filing that they expect him to lose consciousness within seconds and die in a matter of minutes.
On Wednesday, the Supreme Court declined to hear his legal challenge – and also denied Smith’s request to stay his execution. No justice publicly dissented from the ruling.
Smith had also made a separate legal challenge to the lower 11th US Circuit Court of Appeals – where he contested the legality of Alabama’s nitrogen gas protocol.
But that court also rejected Smith’s request to stop the execution, in a ruling on Wednesday evening.
Smith’s lawyers are expected again to appeal to the Supreme Court for a last-minute reprieve.
Smith’s victim, Mrs Sennett, was stabbed and beaten with a fireplace implement, and her death was staged to look like a home invasion and burglary.
Her husband, a debt-ridden preacher, had orchestrated the scheme to collect insurance money. He killed himself as investigators closed in on him.
Smith’s fellow hitman, John Forrest Parker, was executed in 2010.
Alabama is one of three US states that have approved the use of nitrogen hypoxia as a method of execution.
Alabama Attorney General Steve Marshall previously called the method “perhaps the most humane method of execution ever devised”.
The state has one of the highest per capita execution rates in the US and has 165 people currently on death row.
Since 2018, the state has been responsible for three botched attempts at lethal injection in which the condemned inmates survived. The failures led to an internal review that largely placed blame on the prisoners themselves.