The death penalty is "inadmissible" and has no place in modern systems of justice, according to Pope Francis.

The pope made his pronouncement in a letter to the president of the International Commission Against the Death Penalty, Federico Mayor, at the Vatican on Friday. Yesteday it was decided that the American state of Utah would resume death by firing squad for capital punishment when lethal injections are not available. The provision reflects the difficulty of sourcing the chemicals need for lethal injections as they are made in European countries whose governments ban their use for such a purpose.

Pope Francis said in his letter there were no circumstances in which the death penalty was appropriate, called it "cruel, inhuman and degrading" and referred to the "terrible waiting" between sentencing and execution. Efforts to make the sentence more humane were futile, he said, because "There is no humane way of killing another person."

"Today the death penalty is inadmissible, no matter how serious the crime of the condemned," he said. "It is an offence against the inviolability of life and the dignity of the human person that contradicts God's plan for man and society and his merciful justice, and it impedes fulfilling the just end of the punishments. It does not do justice to the victims, but foments vengeance."

He said that capital punishment could only be about vengeance, not about rehabilitation. "When the death penalty is applied, persons are killed not for present aggressions, but for harm caused in the past. Moreover, it is applied to persons whose capacity to harm is not present but has already been neutralised, and who find themselves deprived of their freedom," he said.

He also argued that "The death penalty loses all legitimacy given the defective selectivity of the criminal system and in face of the possibility of judicial error. Human justice is imperfect, and not to recognise its fallibility can turn it into a source of injustices."

Even life imprisonment, he said, was a form of "covert death penalty" as it robbed people of hope.

In his letter, Pope Francis was returning to deeply held convictions he has expressed before. "It is impossible to imagine that states today cannot make use of another means than capital punishment to defend peoples' lives from an unjust aggressor," he said last October in a meeting with representatives of the International Association of Penal Law.