The state supreme court on Monday ruled unanimously to grant a stay of execution to Warren Hill, saying it needed time to decide whether a recent change in execution method complied with state law.

Hill, a 52-year-old African-American, was to be the first person in Georgia to be executed using a single drug, pentobarbital, instead of the previously standard three-drug mixture.

The court may need weeks to decide whether the new execution method, which had been approved by a lower court, is legal under Georgia law.

His execution was scheduled for Monday evening local time.

Hill has spent the last 21 years of death row, sentenced to death for fatally bludgeoning his prison cellmate in 1990.

He had already been serving a life sentence for murdering his girlfriend in 1986.

Criteria for mental disability

Hill's lawyers have argued that he is ineligible for capital punishment, as his reported IQ of 70 puts him below the threshold of mental disability.

The US Supreme Court ruled in 2002 that prisoners with mental disabilities should not be executed, but left each state to arbitrate on what constitutes mental disability.

Georgia requires proof of mental disability "beyond a reasonable doubt," while all other 49 states consider "a preponderance of evidence" to be sufficient.

Accordingly, a Georgia judge ruled last week that Hill was not intellectually disabled enough to preclude the death penalty.

Several high profile figures, including former US president Jimmy Carter, as well as the family of Hill's victim, have called for Georgia to commute Hill's sentence to life in prison.

France has also called for the execution to be suspended, as has a United Nations human rights expert.

tj/rc (AFP, dpa)