The life of Troy Davis, a convicted murderer at the centre of one of the most protracted and controversial death penalty cases in US history, has entered what is expected to be its final hours, after a panel in Atlanta rejected his last-ditch plea for clemency.

At around 7pm local time (midnight GMT) tomorrow, 42-year-old Davis will be escorted to an execution chamber at the Georgia Diagnostic and Classification Prison outside the city of Jackson. He is scheduled to be killed by lethal injection.

The execution comes after Georgia’s five-man Board of Pardons spent Monday hearing a plea for his sentence to be commuted. Supporters argue that Davis’s conviction for the murder of off-duty police officer Mark MacPhail in 1989 was deeply flawed.

The Board heard submissions from several jurors who found Davis guilty but have since changed their mind, along with a selection of key prosecution witnesses who have recanted the evidence they gave at his trial in 1991. A spokesman refused to elaborate on why the board rejected his latest plea, or by what majority the decision was reached.

"I am utterly shocked and disappointed at the failure of our justice system at all levels to correct a miscarriage of justice,” said Brian Kammer, a lawyer for Davis. Reverend Al Sharpton, the civil rights activist, called the decision “one of the most egregious examples of injustice I have seen in years.”

The State of Georgia does not allow its Governor to intervene in capital cases, meaning that only two likely avenues remain open to Davis’s legal team in the run-up to tomorrow's execution: they must either convince the Board to reconsider, or persuade the District Attorney to withdraw the execution warrant.

Both are at best longshots. A third option would be for Davis to persuade the US Supreme Court to intervene. However his lawyers successfully used the Supreme Court to stave off a scheduled execution in 2008, and they believe it is highly unlikely to intervene in the same case twice.

Supporters of Davis argue that there is no physical evidence to support his conviction for shooting MacPhail, who was shot and killed after intervening to prevent a homeless man being beaten outside the store where he worked nights as a security guard. No murder weapon was ever found.

Seven of the nine prosecution witnesses who testified in court that they saw Davis carry out the attack have since recanted, saying they were pressured into giving evidence by investigators. Two further witnesses have since come forward saying that another man, Sylvester Coles, confessed to the crime.

The case has particular resonance in the Deep South, where a hugely disproportionate number of men on death row come from ethnic minorities. Davis has been characterised by supporters as an innocent black man who was hastily arrested and charged by white investigators, and wrongly convicted by a mostly white jury.

Demonstrators are gathering outside the prison where he will be put to death, while more than a million people have now signed a petition protesting his innocence. His execution is opposed by Amnesty International, the European Union, and the National Association for the Advancement of Colored People.

During the two decades since his conviction, Davis has been given no less than four execution dates, all of which have been cancelled at the last minute. In 2008, the US Supreme Court intervened two hours before he was scheduled to be killed, ordering a judge in Georgia to reconsider his conviction. However that judge, William T Moore, last year refused to grant clemency.

The family of MacPhail has meanwhile led calls for Davis to be put to death, saying that they have no doubt as to his guilt. The victim’s mother, Anneliese MacPhail, yesterday told CNN: “I will never have closure, but I may have some peace when he is executed.”