Convicted murderer Warren Hill will die on Monday evening, barring a last-minute intervention by the U.S. Supreme Court, after Georgia's Board of Pardons refused the mentally handicapped inmate's request for clemency late last week.

Hill, who has a reported IQ of 70, has been on Georgia's death row for 21 years for the 1990 killing of Joseph Handspike, a fellow inmate at a prison where Hill was already serving a life sentence for the murder of his girlfriend. While a Georgia judge last week ruled that a preponderance of evidence indicates Hill suffers from mental retardation -- thus making it illegal to apply capital punishment -- the prisoner's motion for a stay of execution was denied because Hill's disability was not proved beyond a reasonable doubt.

The Supreme Court in 2002 ruled the execution of mentally disabled inmates violates the U.S. Constitution's ban on cruel and unusual punishment,but left each state with the authority to determine what constitutes a mental disability. Georgia is the only state in the nation that requires a defendant to prove mental retardation beyond a reasonable doubt, the heaviest proof of burden in the law.

Although the nation's high court declined to review Hill's case, Agence France-Presse reports Hill's lawyers have requested the matter be reconsidered.

The upcoming execution has resulted in wave of protests from Human Rights Watch, the United Nations and Georgians For Alternatives to the Death Penalty, who are urging the state of Georgia to commute Hill's sentence to life in prison without parole.

In a statement, Christof Heyns, the UN Special Rapporteur on arbitrary executions, expressed concern that Georgia's burden of proof requirement could wrongfully lead to the death of severely handicapped individuals, and called on the United States to demonstrate moral and legal leadership by commuting Hill's sentence.

The higher standard of proof, making it very difficult to demonstrate that one actually suffers from a psychological disability may, I fear, mean that Mr. Hill ... would be a fatality in violation of international as well as domestic law, Heyns said.

Former President Jimmy Carter, as well as the family of Hill's victim, have called on Georgia to commute the defendant's sentence. France has also expressed its concern about the situation, asking the U.S. to halt the execution as a first step toward a universal moratorium on capital punishment.

If the sentence is carried out as planned, Hill will be the first inmate in Georgia put to death using only one drug, instead of the three-drug cocktail the state has been using since 2008. Last Tuesday, the day before Hill was initially scheduled to die, the prison system announced it was abandoning its usual drug protocol -- a sedative followed by the paralytic pancuronium bromide and then potassium chloride, which stops the heart -- to using a single drug, the barbiturate pentobarbital.

The Department of Corrections will present its new drug protocol before a Fulton County Superior Court judge on Monday, the Atlanta Journal-Constitution reports. If the judge finds fault with the state's new method of execution, Hill's death sentence could be postponed, yet again.