The convicted killer is Roger Keith Coleman, who died in Virginia's electric chair in 1992 for the rape and murder of his sister-in-law a decade earlier. Until the moment of his execution, Coleman insisted he was innocent.

The case attracted huge attention at the time but Virginia's then governor Douglas Wilder allowed the execution to go ahead after Coleman failed a last-minute lie detector test.

Despite an unrelenting campaign by Coleman's supporters, the Virginia authorities have refused every demand for DNA re-testing with more sophisticated techniques than those available when Coleman was executed.

But Mark Warner, the outgoing governor, reversed those rulings.

"Technology has advanced significantly, and can be applied in the case of someone who consistently maintained his innocence until execution, " he said.

"We must always follow the available facts until a more complete picture of guilt or innocence [emerges]."

Previous tests have indicated that Coleman was among a 2% group of the US population that could have committed the crime. A hair on the victim's body was identified as similar to his, while he had a previous conviction for attempted rape.

Results of the new tests could be available before Warner's term expires next weekend.

The outcome could lay to rest doubts about one of the most controversial executions in modern times. Or it could provide opponents of capital punishment with what they have always wanted: proof that an innocent man has been put to death.