Judge Emily Marks had previously allowed the execution to proceed, arguing that no execution is entirely without pain.

A federal judge in the United States has permanently blocked Alabama from executing an inmate with nitrogen gas, after declaring that the method violates the ban on cruel and unusual punishment.

On Tuesday, US District Judge Emily C Marks permanently enjoined the state from executing Jeffery Lee by nitrogen gas. Lee was scheduled to be executed Thursday at an Alabama prison.

Her decision came a day after an appeals court reversed her earlier ruling that the method is constitutional.

The case centres on how to interpret the US Constitution’s Eighth Amendment, which bars the government from inflicting “cruel and unusual punishments”.

A spokesman for Alabama Attorney General Steve Marshall said the state is reviewing the decision and considering next steps, including an appeal. The case will likely end up before the US Supreme Court, which has previously let nitrogen executions proceed.

A spokeswoman for Lee’s legal team said they did not have an immediate comment.

In her 26-page ruling, Marks said litigation is a constant in death penalty cases.

“Were Alabama to adopt firing squad as a method of execution, that method would likely be challenged as well. Indeed, there is likely no method — no matter how humane — that would be immune to constitutional challenge,” Marks wrote.

“But the Constitution does not guarantee a painless death, and human life cannot be purposefully extinguished without some risk of pain. The Court, the condemned, and the State must all confront that sobering reality.”

Marks noted that the state has two other authorised execution methods: lethal injection and the electric chair. She said Lee is “not entitled to an injunction barring the State from executing him using one of those methods”.

Marks also ruled that the state could switch to Lee’s preferred method, a firing squad. Inmates challenging execution methods are required to suggest an alternative method.

“The State can readily obtain rifles, ammunition, and other materials necessary to carry out a firing squad execution,” Marks wrote.

“Additionally, the State would be able to modify space at Holman to carry out executions by firing squad. The State is also able to source and train volunteers willing to carry out such an execution.”

Lee is currently housed at Holman Correctional Facility in Atmore.