Pope Francis has branded life-long prison terms “a hidden death sentence” in an attack on “penal populism” that included severe criticism of countries that facilitate torture.

In a wide-ranging speech to a delegation from the International Association of Penal Law, the pontiff said believers should oppose life-long incarceration as strongly as the use of capital punishment.

“All Christians and men of good faith are therefore called upon today to fight, not only for the abolition of the death penalty – whether it is legal or illegal and in all its forms – but also to improve the conditions of incarceration to ensure that the human dignity of those deprived of their freedom is respected.

“And this, for me, is linked to life sentences. For a short time now, these no longer exist in the Vatican penal code. A sentence of life (without parole) is a hidden death penalty.”

In comments likely to enhance his reputation as one of the most liberal of popes, Francis went on to slam the risk of sentencing becoming disproportionately severe.

“In recent decades a belief has spread that through public punishment the most diverse social problems can be resolved, as if different diseases could all be cured by the same medicine.”

Reiterating Catholic teaching that capital punishment is a sin, the pope also made what appeared to be a thinly veiled attack on the European countries which have facilitated US demands for extraordinary rendition of terror suspects to detention centres in parts of the world where they can be tortured with impunity.

“These abuses will only stop if the international community firmly commits to recognising … the principle of placing human dignity above all else.”