John Paul Penry's case is also being considered

The judges will also hear arguments by lawyers for John Paul Penry, awaiting execution in Texas, that jurors were not given the chance to assess his mental capacity.

The court used Penry's case in 1988 to rule that mentally retarded murderers could be executed.

McCarver, aged 40, was convicted of stabbing and choking to death a 71-year-old colleague at a cafe where he worked.

'Cruel and unusual'

But his lawyers say that he is mentally retarded and has the mind of a 10-year-old.

They maintain that his execution would be a violation of the US Constitution's 8th Amendment ban on cruel and unusual punishment, citing what they describe as a "newly evolved consensus against executing the mentally retarded".

Lawyers for the prosecution deny that McCarver is retarded, and say that, even if he is, he should still be executed.

The court halted McCarver's execution on 1 March after he had eaten his last meal.

North Carolina Governor Mike Easley had already rejected his clemency petition on the grounds that he had planned and orchestrated the killing and had been sufficiently competent to gain employment.

Amnesty International has called on the US to ban the death penalty.

It describes execution as "a cruel, brutalising, unreliable, unnecessary and hugely expensive activity for no measurable gain".

Twelve US states do not have capital punishment and another 13 prohibit the execution of mentally retarded killers.

More than 700 people have been executed in the US since the death penalty was re-introduced in 1976, including 85 last year.