A prisoner, who won a landmark case in which the US Supreme Court ruled that mentally retarded prisoners could not be executed, could find himself back on death row because his IQ has improved as a result of working on his defence.

A prisoner, who won a landmark case in which the US Supreme Court ruled that mentally retarded prisoners could not be executed, could find himself back on death row because his IQ has improved as a result of working on his defence.

In an extraordinary reversal of fortune, Daryl Atkins will return to court later this year where a jury will decide whether he is officially retarded. If the court decides he is not, Atkins, 27, could be executed by lethal injection ­ in effect, because of the work he did that resulted in other mentally retarded prisoners being spared.

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Atkins' IQ was first tested in 1998 and was found to stand at 59. But when it was tested more recently, he scored 76. In Virginia, the level at which the state differentiates between someone who is retarded and someone who is not is 70.

A report about Atkins recently completed by Evan Nelson, a psychologist who was hired by the defence, said: "Oddly enough, because of his constant contact with the many lawyers that worked on his case Mr Atkins received more intellectual stimulation in prison than he did during his late adolescence and early adulthood.

"That included practising his reading and writing skills, learning about abstract legal concepts and communicating with professionals."

Defence lawyers argued last week, before a court hearing in Yorktown, that his IQ at the age of 18 ­ when he committed the crime for which he was sentenced to death ­ was much lower. It was unfair, they argued, to judge him according to his current mental competence now than by the situation when he was convicted.

Atkins was convicted and sentenced to death for abducting and murdering 21-year-old Eric Nesbitt in 1996. Atkins and another man seized Nesbitt, an airman from Langley Air Force Base, and forced him to withdraw money from a cash machine before shooting him dead. The murder was recorded on closed-circuit television.

In 2002, the Supreme Court saved Atkins when it ruled 6-3 that the execution of retarded prisoners was unconstitutional because it breached the 8th Amendment which prohibits "cruel and unusual" punishments. Dozens of mentally retarded prisoners were released from death row as a result.

Justice John Paul Stevens wrote at the time: "Because of their disabilities in areas of reasoning, judgement, and control of their impulses ­ they do not act with the level of moral culpability that characterises the most serious adult criminal conduct. Moreover, their impairments can jeopardise the reliability and fairness of capital proceedings against mentally retarded defendants." He added: "We are not persuaded the execution of mentally retarded criminals will measurably advance the deterrent or the retributive purpose of the death penalty."

The court had previously declared the execution of the mentally handicapped legal. But since then, the number of states which have the death penalty that have banned capital punishment for inmates deemed mentally disabled has risen from two to18.

In Atkins' case, prosecutors have questioned whether he was ever truly retarded. The state's lawyer, Eileen Addison, said that his ability to load and use a gun, and to recognise a cash machine, to direct Mr. Nesbitt to withdraw money and to identify a remote area for the killing showed that he was not retarded and proved his competence.

Since 2002, various states across the US have been struggling to interpret the Supreme Court's ruling. According to the Death Penalty Information Centre, most have required defendants to show three things: that their IQ is below 75 or 70, that they lack basic social skills and practical abilities, and that the conditions existed before they turned 18 years of age.

David Gossett, a lawyer who represents a prisoner in Georgia in a similar position to Atkins, said: "Prisons are highly structured environments. They're sometimes good environments for the mentally retarded. These people are not vegetables ­ they can learn. They are people who can get better at taking tests."