Former US president Jimmy Carter has called for a new nationwide moratorium on the death penalty, arguing that it is applied so unfairly across the 32 states that still have the death sentence that it amounts to a form of cruel and unusual punishment prohibited under the US constitution.

In an interview with the Guardian, Carter calls on the US supreme court to reintroduce the ban on capital punishment that it imposed between 1972 and 1976. The death penalty today, he said, was every bit as arbitrary as it was when the nine justices suspended it on grounds of inconsistency in the case of Furman v Georgia 41 years ago.

“It’s time for the supreme court to look at the totality of the death penalty once again,” Carter said. “My preference would be for the court to rule that it is cruel and unusual punishment, which would make it prohibitive under the US constitution.”

Carter’s appeal for a new moratorium falls at a time of mounting unease about the huge disparities in the use of capital punishment in America. Recent research has shown that most of the 1,352 executions that have taken place since the supreme court allowed them to recommence in 1976 have emanated from just 2% of the counties in the nation.

Amid a critical shortage of medical drugs used in lethal injections caused by a European boycott of US corrections departments, death penalty states are also adopting increasingly desperate execution methods. The new techniques range from deploying previously untested sedatives in lethal injections, to concocting improvised batches of the chemicals through compounding pharmacies.

Carter’s main concern is what he sees as the injustice in the kinds of individuals who are most likely to be sentenced to death and executed. He will be raising his concerns at a national symposium on the death penalty held by the American Bar Association and hosted by his Carter Center in Atlanta, Georgia on Tuesday.

“The only consistency today is that the people who are executed are almost always poor, from a racial minority or mentally deficient,” he told the Guardian. “In America today, if you have a good attorney you can avoid the death penalty; if you are white you can avoid it; if your victim was a racial minority you can avoid it. But if you are very poor or mentally deficient, or the victim is white, that’s the way you get sentenced to death.”

He added: “It’s almost inconceivable in these modern days to imagine that a rich white man would be executed if he murdered a black person.”

Carter’s call for a renewed moratorium is particularly significant given his role, paradoxically, in helping to restore the death penalty in the 1970s. In 1973, as governor of Georgia, he signed into law a new set of guidelines that were designed to meet the objections of the supreme court over the inconsistencies in the death penalty set out in the Furman case.

Those new guidelines were reviewed by the supreme court, and on the back of that case – Gregg v Georgia – the nine justices removed their previous injunction and allowed the death penalty to resume. Carter now looks back on the part he played with frank regret.

“If I had to do that over again I would certainly be much more forceful in taking actions what would have prohibited the death penalty,” he said, referring to his signing of the 1973 guidelines. “In complete honesty, when I was governor [of Georgia] I was not nearly as concerned about the unfairness of the application of the death penalty as I am now. I know much more now. I was looking at it from a much more parochial point of view – I didn’t see the injustice of it as I do now.”

In recent years Carter has striven to make up for what he sees as his past weakness on the issue in his home state, which has carried out some of the most controversial judicial killings in modern times. He campaigned vociferously against the execution of Troy Davis in September 2011 amid serious doubts about the prisoner’s guilt .

He continues to campaign in the case of Warren Hill, a Georgia death row inmate who has been declared intellectually disabled (or “mentally retarded” in US jurisprudence) by all the psychiatrists who have examined him, yet still faces execution despite a US supreme court ban on executing intellectually disabled people. “I would hope Georgia would commute Warren Hill’s sentence completely,” Carter said.

At Tuesday’s symposium, the ABA will present the findings of its eight-year examination of the fairness and accuracy of several of the leading death penalty jurisdictions in the US. It has carried out a series of assessments of the way capital punishment is applied in Alabama, Arizona, Florida, Georgia, Indiana, Kentucky, Missouri, Ohio, Pennsylvania, Virginia, Tennessee, and Texas – states that represent 65% of all executions that have taken place since 1976.

The ABA has identified 12 areas in which the practice of capital punishment in the US continues to experience serious problems. They include the use of ill-equipped, poorly trained and under-paid lawyers to defend people facing capital prosecutions; racial disparities in the pursuit of cases – in Georgia, those suspected of killing whites are almost five times more likely to be sentenced to death than those suspected of killing blacks; huge variations in the rules relating to DNA testing; widespread confusion among jurors sitting at death penalty trials; and the fact that all the assessed states continue to allow people with severe mental illness to be sentenced to death and executed.

Ed Pilkington will be acting as a moderator at the American Bar Association symposium at the Carter Center on Tuesday