A Georgia parole board has decided to reject a request for clemency for death row inmate Troy Davis after it has heard testimonies from supporters and prosecutors.

Georgia Board of Pardons and Paroles spokesman Steve Hayes has said the panel decided to not grant the request after it heard hours of testimony.

Davis, 42, is expected to undergo lethal injection on Wednesday.

The death row inmate was convicted of slaying an off-duty officer Mark MacPhail in 1989. The police officer was moonlighting as a security guard outside a Savannah bus station on Aug. 19 when he was shot and killed. He was responding to the cries of a homeless man who was being pistol-whipped in a Burger King parking lot.

Davis' 1991 conviction was based on the testimony of nine witnesses who said they'd seen Davis pull the trigger. However, Davis, who was present at the scene, has said he is innocent, and that another man, Sylvester Coles, attacked the homeless man and also shot MacPhail when he tried to mediate.

A murder weapon was never found and there was no DNA or other forensic evidence, according to the Guardian. Also seven of the nine witnesses have come forward since the trial and recanted their testimony. They said police coerced them into implicating Davis.

There are others who have said Coles have confessed to killing the cop.

Davis' execution will go through as planned on Wednesday, as it seems he has exhausted all avenues that could prevent his judicial killing.

I am utterly shocked and disappointed at the failure of our justice system at all levels to correct a miscarriage of justice, Brian Kammer, one of Davis's attorneys, to the UK Guardian following the pardons board's decision.

But the victim's mother Anneliese MacPhail has said Davis' death would bring her peace.

I think I finally will have peace of mind, she said. When it is over I can close that book and I know Mark can rest in peace, too.

Human rights group such as Amnesty International and the NAACP have called on the pardons board to reconsider their decision. It is the fourth time in four years that Davis' execution has been scheduled by Georgia officials, The AP said.

This is a huge setback for human rights in the USA, where a man who has been condemned under dubious evidence is to be executed by the state, said Salil Shetty, Amnesty International's Secretary General, in a statement. Even at this late stage, the board must reconsider its decision.

Shetty also said the decision by Georgia's Board of Pardons and Paroles to reject Troy Davis' appeal for clemency is at odds with its 2007 decision when they counseled against execution if there was 'doubt as to the guilt of the accused.'

Even if members of the Board were convinced that there was no doubt, many other people have not been so persuaded, Shetty said. Clearly, the U.S. capital justice system is capable of making mistakes. The persistent doubts that have plagued the Troy Davis case point to a fundamental flaw of the death penalty. It is irrevocable - and in the USA, the death penalty is also marked by arbitrariness, discrimination and error.