Legal battles and the shortage of drugs needed for lethal injection meant just five of 31 US states executed inmates.

The number of executions in the United States fell to a quarter-century low in 2016 as new death sentences plummeted, indicating that capital punishment is on the decline, a study released on Wednesday showed.

Twenty people were executed in the US this year – the lowest since 1991, according to the study from the Death Penalty Information Center, which monitors capital punishment.

While 31 US states have the death penalty, only five carried out executions in 2016. Georgia did the most with nine, while Texas was next at seven, it said.

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The number of new death sentences handed down in 2016 is expected to hit 30, a low not seen since the US Supreme Court declared existing death penalty statutes unconstitutional in 1972, it said.

That figure is down by more than 90 percent from a recent high of 315 in 1996.

Legal battles and a sales ban on execution drugs will likely hold down the number of state-sponsored killings next year, while the high costs of death penalty cases is set to keep capital punishment prosecutions down as district attorneys instead seek sentences of life in prison without parole, legal experts said.

“America is in the midst of a major climate change concerning capital punishment,” said Robert Dunham, the centre’s executive director and the report author.

Drug shortage stalls lethal injections

States have been scrambling to find execution drugs since European drug makers imposed a sales ban about five years ago over ethical concerns. The problem was exacerbated when pharmaceutical giant Pfizer imposed a sales ban this year, cutting off the last major US source of the killer drugs.

Ohio, which has executed 53 inmates since the US Supreme Court reinstated the death penalty in 1976, had a US judge this week delay plans to end its nearly three-year execution hiatus in 2017 to examine its drug procurement secrecy.

Capital punishment advocates have said expenses or drug shortages should not be a factor, arguing the death penalty is an instrument of justice and must used for those who deserve it.

Jordan Steiker, a University of Texas Law School professor and director of its Capital Punishment Center, said states looking to resume executions are going to face stiff legal challenges.

“We are on a path toward constitutional abolition. The length of that path will be dictated by uncertainties concerning the Supreme Court’s composition and how much the withering of the death penalty continues,” he said.

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However, the number of executions in the US could jump dramatically in the coming years depending on how a legal battle in California ends.

A ballot initiative approved by voters to speed up death penalty appeals was put on hold on Tuesday by the California Supreme Court to consider a lawsuit challenging the measure.

The court issued a one-page decision staying the “implementation of all provisions of Proposition 66” and set a timeline for filing briefs that the court will consider before deciding to hold a hearing.

Proposition 66 would change how appeals are handled with the goal of expediting them so murderers are actually put to death. There hasn’t been an execution in California for more than a decade, and 750 inmates remain on death row.

California has the highest number of death row inmates in the country.

The measure would appoint more lawyers to take cases, putting certain types of appeals before trial court judges and setting a five-year deadline for appeals to be heard.

Currently, it can take longer than that for an attorney to be assigned to a case and more than 25 years to exhaust appeals.

California voters faced two death penalty measures in the November election.

They rejected a measure that would have abolished it, and narrowly approved Proposition 66. That initiative also included a provision that would funnel a large portion of inmates’ meagre income to victims.