The supreme court in Georgia has given a temporary 11th-hour reprieve to a mentally handicapped man who was due to be executed - but not for the reason his case attracted attention.

Warren Hill, 52, was scheduled to be put to death by lethal injection for killing a fellow inmate but judges delayed the execution.

Hill, who is reported to have an IQ of 70, which puts him below the threshold for mental disability, was granted a stay of execution 90 minutes before he was due to be injected.

Hill has spent 21 years on death row for killing fellow prisoner Joseph Handspike. Hill was serving time for murdering his 18-year-old girlfriend.

The US supreme court ruled against the execution of prisoners with mental disabilities in 2002, saying the disability "would run the risk of a wrongful execution". However, the court left each state with the authority to determine what constitutes mental disability.

The Georgia court unanimously decided to postpone the death sentence of Hill - which has sparked protests around the world - not because of his mental disability, but because of a dispute over the method that would be used.

Hill was to be the first person in Georgia to be put to death using a single drug, pentobarbital, instead of the previously standard three-drug cocktail.

The court said it needed to investigate whether a lower court erred in determining that the change in execution protocol was legal under Georgia law.

Death Penalty Information Centre director Richard Dieter told AFP: "Georgia, almost on the eve of the execution, switched to a different method, pretty radically different."

Dieter added that there have been "only a handful of executions with this one drug".

"If you want to use it, you need to know a lot more," he said.

The case of Hill also calls into question Georgia's criteria for defining who is mentally disabled before they can be executed.

Georgia is the only state in the US that says a prisoner appealing against a death sentence on the grounds of mental retardation must prove beyond a reasonable doubt that they are retarded - a requirement that is almost impossible to achieve.

James Elis, one of Hill's lawyers, said: "The problem is that Georgia makes the proof requirement essentially impossible to meet for defendants who clearly have mental retardation."

Former US president Jimmy Carter, as well as the family of Hill's victim, have called for his sentence to be reduced to lifetime imprisonment.

Georgia was mired in controversy over the execution of Troy Davis.

Davis was convicted of the murder of police officer Mark MacPhail in Savannah, Georgia, in 1989. He was executed in September despite large amounts of evidence casting doubt on his conviction, including recantations from seven of the nine key witnesses at his 1991 trial.