California's Governor, Arnold Schwarzenegger, has denied clemency to the oldest prisoner on California's Death Row, saying a murderer's life should not be spared because he is old and ill. Although Clarence Ray Allen still has an appeal pending before the Supreme Court, the decision increases the likelihood that he will be executed by lethal injection on Tuesday, a day after he turns 76.

With Allen legally blind, hard of hearing, confined to a wheelchair by the debilitating effects of diabetes, and barely able to speak above a whisper, his judicial killing is being denounced as an affront to human dignity.

His case, the latest in a long line to raise disturbing questions about the way capital punishment is administered in the United States, is filled with ghoulish ironies. He almost died of a heart attack four months ago but doctors at San Quentin prison resuscitated him - fulfilling their professional obligations, just as the prison's executioners are now preparing to fulfil theirs at one minute after midnight on Tuesday.

Special arrangements will have to be made to get Allen into the death chamber, which does not permit wheelchair access because of a steep bump running across the floor. (The chamber was designed for death by poison gas, and the lip helps to make the room airtight.) His lawyers have requested for him to be allowed to take his final steps with a walker, but the prison authorities have not made clear whether they will assent, as protocol dictates that prisoners' hands must be manacled and their feet shackled as they make their final journey from holding cell to death chamber.

Two prison guards will be on hand - either to help Allen drag his feet over the lip of the door or else to carry him bodily on to the stretcher where he will be injected with drugs to knock him out, collapse his lungs and stop his heart.

Even by the grim standards of other executions, his treatment strikes activists as particularly shocking. "The death penalty is never right," Amnesty International's UK campaigns director, Stephen Bowen, said, "but in the case of a seriously ill, elderly man with possible brain damage it is an affront to all standards of decency and justice."

Mr Schwarzenegger turned down Allen's petition for clemency on Friday, just as he did a month ago in the case of Stanley "Tookie" Williams, one of the founders of the Crips street gang who became an outspoken and widely admired anti-gang activist in his final years on Death Row. That decision sparked an international furore.

There is little doubt about Allen's guilt. He organised robberies under the guise of a security company he ran in Central Valley in the 1970s, then arranged the murder of four people who snitched to the authorities. The last three murders were ordered after he was behind bars.

But he hardly poses any threat now. Among those who support clemency are Daniel Vasquez, the former warden of San Quentin, who called him "a pathetic sight - aged, downcast, dejected, isolated, oblivious to his surroundings, cuffed to his wheelchair, and utterly defeated".