Warren Hill, a death row prisoner in Georgia who has been diagnosed intellectually disabled, has been granted a stay of execution 90 minutes before he was scheduled to be put to death by lethal injection.

The Georgia supreme court unanimously decided to postpone the death sentence in a case that has caused an uproar nationally and around the world.

Hill was set to be executed despite the fact that the US supreme court has banned executions for "mentally retarded" criminals on grounds that they constitute cruel and unusual punishment.

The stay of execution was ordered on the basis of a dispute over the technique of the killing, not the fact that it was to be carried out in itself. The state has recently moved from a three-drug lethal injection procedure to a single fatal dose of the sedative pentobarbital.

The judges said they would now consider whether the switch in procedures amounts to a violation of Georgia's administrative procedure act that governs executions.

The ruling has spared Hill's life for now. But it leaves hanging in the air the question of whether Georgia should be allowed to execute "mentally retarded" prisoners despite the US supreme court ban.

Georgia is the only state in the country to apply a standard of "beyond a reasonable doubt" in cases of intellectual disability. That means that death row inmates have to prove beyond a reasonable doubt that they are "mentally retarded" if they are to avoid the death chamber – a requirement that is almost impossible to achieve.

Hill's lawyers have indicated they will continue to challenge Georgia's burden of proof on mental disability to the higher courts.

In 2002, in the case of Atkins v. Virginia, the US supreme court ruled that putting to death a person who was "mentally retarded" was a violation of the eighth amendment of the US constitution that bans cruel and unusual punishment.

"The supreme court issued a clear mandate that 'mentally retarded' prisoners should not be executed, yet Georgia has developed a loophole," said Richard Dieter, who heads the Death Penalty Information Center.

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