The US state of Georgia has rejected a final clemency appeal for Troy Davis, due to be executed for the 1989 murder of an off-duty policeman.
The 42-year-old is due to face a lethal injection on Wednesday evening.
He was convicted of shooting of Mark Allen MacPhail in Savannah, Georgia, but has since attracted support for his claim of wrongful conviction.
Davis was sentenced in 1991, but most of the witnesses have since recanted or changed their testimony.
The spokesman for the Georgia Board of Pardons and Paroles said the panel made their decision after hearing hours of testimony from both supporters and prosecutors.
Davis has been scheduled to be executed four times in the past four years, but defence lawyers have described the appeal to the pardons board as likely to be Davis’ last option.
Prosecutors insist they have no doubt that they charged the right person with the crime.
MacPhail’s relatives were relieved at the parole board decision.
“Justice was finally served for my father,” said Mark MacPhail Jr, the victim’s son. “The truth was finally heard.”
Amnesty International, which has supported Davis in his wrongful conviction claim, called decision “unconscionable”.
“Should Troy Davis be executed, Georgia may well have executed an innocent man and in so doing discredited the justice system,” Amnesty International USA director Larry Cox said in a statement.
Over one million people worldwide have signed petitions for clemency in his case.
Pope Benedict XVI, former US President Jimmy Carter and EU foreign policy chief Catherine Ashton are among those who have backed Davis, who has always maintained he is innocent.
On Monday, a vigil took place outside the hearing. Demonstrators held signs that said “I am Troy Davis” and “Save Troy Davis”, and chanted for his release.
Davis’ conviction has been questioned because seven of nine witnesses who helped convict him during the original trial have either backed off their testimony or recanted.
No murder weapon was ever found and no DNA evidence or fingerprints conclusively linked Davis to the shooting.
Davis’ legal team said in a statement they were “incredibly disappointed” by the board’s decision as the prosecution’s case “cannot resolve the significant, lingering doubts that exist here”.
They called on District Attorney Larry Chisolm to vacate the death warrant and for the Board to reconsider their decision immediately.
At a rare 2010 innocence hearing – ordered by the US Supreme Court – two witnesses said they falsely incriminated Davis, while two others told the court another had confessed to being the actual killer.
US District Court Judge William T Moore Jr said there was not enough evidence to vindicate Davis or grant him a new trial.
Federal appeals courts and the Georgia Supreme Court have upheld Davis’ conviction.