The U.S. state of Georgia is scheduled to put to death a convicted murderer, whom rights groups and a United Nations official say should be spared execution because he is mentally disabled.

Warren Hill is set to die Monday for the 1990 murder of a fellow inmate. Already in prison for life in connection with another murder, Hill beat the inmate to death using a two-by-six, nail-studded board. The judge presiding on the last murder acknowledged that, with an I.Q. of 70, Hill was more likely that not to be mentally retarded. However, he said Hill's attorney failed to do that. Federal courts upheld Georgia state law that requires defendants to prove claims of mental retardation beyond a reasonable doubt.

The U.S. Supreme Court last month declined to hear Hill's appeal and last week Georgia's parole board rejected a request to change the sentence to life in prison.

U.N. Special Rapporteur on arbitrary executions Christof Heyns called on the state to show “the moral and legal leadership” expected of the United States. He warned that other governments may follow and impose similar sentences instead of “more humane” punishments.

The Supreme Court ruled in 2002 that it is unconstitutional to execute people who are mentally retarded, saying those defendants “face a special risk of wrongful execution.” The decision left it up to individual states to decide what level of proof defendants must show of their mental capacity.

The 52-year-old Hill will also be the first inmate in Georgia to be executed using a single drug, rather than a three-drug mix the state has previously used. He was originally set to die last week, but the state delayed the execution because of the change in drugs.