Warren Hill, who has come hours away from execution four times in as many years, will be executed at 7pm barring a US supreme court-issued stay

Georgia is hours away from executing an intellectually disabled man, in an apparent violation of a constitutional ban that prohibits cruel and unusual punishment.

Warren Hill is scheduled to be executed by lethal injection on Tuesday at 7pm for killing a fellow prisoner, Joseph Handspike, in 1990. It is the fourth time Hill has come within hours of execution in as many years – a repeated exposure to the death chamber his lawyers have likened to psychological torture.

On Tuesday morning, Georgia’s parole board denied clemency for Hill, saying it had reviewed his criminal record and life history. His last chance now rests with the US supreme court, where a final request for a stay of execution has been submitted.

Warren Hill: 10 reasons why Georgia should not execute him Read more

On the face of it, Hill’s case for being spared the death chamber appears beyond argument. In 2002, the nation’s highest court banned the execution of intellectually disabled prisoners. In Hill’s case, medical opinion is unanimous: every expert who has examined him, including three for the state, have found him to be intellectually disabled.

The snag was that the supreme court left it up to individual states to set the standard of definition of “mentally retarded” (the legal term used) – which allowed Georgia to skirt around the prohibition by setting a uniquely tough standard of “beyond reasonable doubt”. Mental-health experts say it is almost impossible to prove beyond a reasonable doubt that someone is intellectually disabled, because this is a condition that involves a judgment call on the part of experts rather than a hard and fast measurement.

A plethora of national professional bodies have protested that Hill is entitled to constitutional protections. In addition, Georgia’s own courts have twice agreed with the experts and found Hill to be intellectually disabled.

Last year, the US supreme court returned to the subject of how to define “mental retardation” in the wake of confusion in a case in Florida. In Hall v Florida, the court made clear that states did not have a free hand in interpreting the law in ways that flouted constitutional protections.

“The states are laboratories for experimentation,” the supreme court said, “but those experiments may not deny the basic dignity the constitution protects.”

Despite the weight of legal and medical opinion, an intellectually disabled man still faces death by lethal injection in Georgia, to the dismay of death penalty lawyers. Brian Kammer, who legally represents Hill, told the Guardian: “The supreme court needs to police states like Georgia on these issues. Otherwise, the constitution loses its authority and force, and we have a miscarriage of justice.”

Kammer also said that in his view the repeated threat of execution that has brought Hill within a few hours of lethal injection four times in four years was in itself a violation of the constitution.

“This is psychological torture,” he said. “Even more so when you consider that we are dealing with someone with an intellectual disability. Hill has difficulty handling stress, so this has taken a severe toll on him.”

Texas set to execute death row inmate diagnosed as 'mentally retarded' Read more

The issue of prisoners with intellectual disabilities facing execution is doubly prominent this week. On Thursday, Texas is due to kill a prisoner who fits the definition. Convicted murderer Robert Ladd has consistently been diagnosed as having severe intellectual impairments, yet the state and local courts have so far declined to afford him the protections set out in the constitution.

In Texas, the state follows its own bizarre guidelines for how to define “mental retardation”. A prisoner has to show that his intellectual abilities are comparable to those of the character Lennie Small in John Steinbeck’s 1937 novel Of Mice and Men.