A death row prisoner in Georgia who has been officially deemed by the courts to be "mentally retarded" is scheduled to be executed next week despite a supreme court ruling that bans the death sentence for people with learning difficulties.

The Georgia State Board of Pardons and Paroles was due to hear a clemency appeal on behalf of the prisoner, Warren Hill, on Friday and has the power to commute his death penalty to life without parole.

But should the five-member board decide to dismiss his plea, Hill will be executed by lethal injection at 7pm on Wednesday at the Georgia Diagnostic and Classification Prison in Jackson in a move that could pit Georgia against the clear will of the supreme court, the highest judicial panel in the nation.

"We are heading into a constitutional crisis," Hill's lawyer, Brian Kammer, said. "The supreme court banned executions of mentally retarded prisoners, but here we are in Georgia about to execute a man who is mentally retarded."

Hill, 52, was sentenced to death for killing a fellow prisoner, Joseph Handspike, in 1990. At the time he was already serving a life sentence for murdering his girlfriend, Myra Wright.

Neither of the juries at his two trials were told that Hill had an IQ of about 70, that he came from a violent home or that from a very young age had displayed signs of learning disabilities. In November 2002 a judge in a Georgia court found Hill to be "mentally retarded" – the label for learning disabilities that is still used widely in the US among official circles.

The judge's finding was made just a few months after the supreme court, in Atkins v Virginia, ruled that executions of individuals with learning disabilities violated the eight amendment of the US constitution that prohibits cruel and unusual punishment.

Despite the supreme court ban, Hill has fallen into a legal trap that could send him to his death next Wednesday. Georgia is the only state in the country that requires prisoners seeking to avoid execution to prove beyond reasonable doubt that they are "mentally retarded".

In the November 2002 judgement, Hill was deemed "mentally retarded" according to a lesser standard of "a preponderance of the evidence". That standard applies in almost all other parts of the country, yet in Georgia it is considered insufficient to spare Hill's life.

"This case highlights the stark injustice in Georgia's capital punishment system," Kammer said. "The question is, does a supreme court ruling count for anything in this state, or is it a paper tiger?"

Paradoxically, Georgia was the first state in America to ban executions of people with learning disabilities. The move was forced by a massive public outcry after the execution in 1986 of Jerome Bowden, who days before he died was tested and found to have the mental capacity of a 12-year-old.

But in passing the ban in 1988, Georgia adopted the "beyond reasonable doubt" standard that remains to this day and now holds Hill's fate in the balance.

The prisoner's appeal for clemency has been backed by many prominent individuals and groups in Georgia, including one of its most famous resident, former president Jimmy Carter. Several disability groups have joined the chorus, as has the family of Hill's second victim.

Richard Handspike, nephew of Joseph Handspike, speaking for the family, said that the death penalty should not have been imposed. "I and my family feel strongly that persons with any kind of significant mental disabilities should not be put to death. I believe that if the system had evidence of such a disability in Mr Hill, it should have taken steps to treat him accordingly and prevent his execution."

Disability groups have warned that Hill's mild form of learning disabilities makes him particularly vulnerable to wrongful execution because their difficulties are not easily identified. Hill had learning challenges from a young age; his school teachers have testified that he had trouble reading and writing and was very withdrawn.

He coped badly with stress. At the time he bludgeoned Handspike to death, he was being held in a shared prison dormitory and was coming under physical and sexual harassment from other inmates.

"He couldn't get away from the harassment. The incident was impulsive – he didn't intend to kill Mr Handspike," Kammer said.