WASHINGTON (AFP) - The execution of a United States inmate diagnosed as "mentally retarded", delayed pending a last-minute appeal, will now take place on Friday, officials in the US state of Georgia said.

The authorities said death-row inmate Warren Hill will be put to death by lethal injection on Friday at 11pm GMT at a prison in the city of Jackson.

They made the announcement in a statement issued on Wednesday.

Hill's execution was postponed on Monday, just hours before it was set to take place.

Hill's attorneys said the stay of execution was related to a defence complaint over what it calls the "extreme secrecy" surrounding the execution.

Hill, 52, was already serving life in prison for the murder of his girlfriend when he fatally bludgeoned a fellow prisoner in 1990. He was subsequently sentenced to death.

A 2002 Supreme Court ruling barred the execution of people suffering from mental impairment. In 2000, three doctors ruled that he did not meet Georgia's criteria - regarded as the toughest in the US - to be classified as mentally disabled.

However, those three doctors have since revised their assessments and unanimously agree that Hill, who has an IQ of 70, is a person with mental retardation. Normally that would be enough to prevent the authorities from executing Hill, but his lawyers complain that they have been "procedurally barred" from raising his case in Georgia's lower courts.

His attorneys say there is one remaining chance for Hill to avoid execution: They were to attend a court hearing on Thursday challenging use of the drug pentobarbital to put him to death.