One of the most hotly contested death row cases in recent years looks set to go ahead with the execution of Troy Davis in Georgia on Wednesday.

Davis lost his final bid for clemency despite overwhelming evidence indicating that his conviction for murder is unreliable.

He will be put on a gurney at the state prison in Jackson and administered a cocktail of lethal drugs at 7pm local time on Wednesday, barring a last-minute intervention by the US supreme court which few observers expect to take place.

The Georgia Board of Pardons and Paroles, which alone has power within the state to commute Davis's death sentence, denied to grant him clemency having heard three hours of testimony on Monday casting deep doubts on his conviction.

Davis, 42, was put on death row 20 years ago for the 1989 murder of a police officer, Mark MacPhail, in Savannah following a fight with a homeless man over a bottle of beer. Since then seven out of the nine key witnesses who implicated him have recanted their evidence, several saying they were cajoled by police into giving false eye-witness statements.

Another 10 have come forward to point the finger at a separate man present at the scene of the murder, Sylvester Coles.

Meanwhile, no forensic or DNA evidence linking Davis to the shooting has ever been found, and nor has the murder weapon.

The denial of clemency by the parole board prompted an outpouring of anger and despair from hundreds of Twitter users and several celebrity supporters of Davis's campaign. The prisoner's lawyer, Brian Kammer, said he was "shocked and disappointed at the failure of our justice system at all levels to correct a miscarriage of justice".

Amnesty International's US branch, that has championed the case, said: "Allowing a man to be sent to death under an enormous cloud of doubt about his guilt is an outrageous affront to justice. The case against Davis unraveled long ago."

The public figures who have leant their names to the "Too much doubt" campaign to have Davis's execution commuted include Archbishop Desmond Tutu, former president Jimmy Carter, the former director of the FBI William Sessions, 51 members of the US Congress and the actor Mia Farrow.

Bianca Jagger, who acts as the Council of Europe's ambassador on the death penalty, said: "to execute Troy Davis in these circumstances would be a travesty. Executing an innocent man is a state-sanctioned murder."

The parole board heard from one of the jurors who originally recommended the death penalty for Davis. Brenda Forrest told the panel she no longer trusted the verdict or sentence: "I feel, emphatically, that Mr Davis cannot be executed under these circumstances," she said, according to the Atlanta Journal-Constitution.

The board also heard from Quiana Glover, who testified she had heard Coles confess in June 2009 to having been the killer, at a party where he had been drinking heavily.

But the family of the murdered police officer have consistently expressed their desire to see Davis executed, saying they have no doubts about his guilt. "A future was taken from me. The death penalty is the correct form of justice," Madison MacPhail, the victim's daughter, said before the parole board announced its decision.

Over the past two decades the Davis case has been subject to an exceptional number of appeals and hearings reaching to the top of the US justice system. Numerous court hearings have dismissed the enormous mound of evidence casting doubt on Davis's conviction as inconclusive.

But the exhaustive nature of previous appeals leads observers to doubt that an eleventh-hour reprieve will be forthcoming.