"Due to time constraints resulting from the delay in the judicial process, the execution was cancelled as it was determined that the convicted inmate's veins could not be accessed according to our protocol prior to the expiration of the death sentence," said Alabama Correctional Services Chief John Hamm, quoted by the US agency AP.

"Access to the veins was taking a little longer than anticipated," Hamm said.

Miller, 57, was taken to his cell in a southern Alabama prison.

In July, Alabama authorities were accused of the botched execution of an inmate after they took more than three hours to start the process because of difficulty administering the lethal injection.

Alabama Governor, Republican Kay Ivey, said that despite today's cancellation, "nothing will change the fact that a jury has heard the evidence in this case" and made the decision to sentence Miller to the death penalty.

A US organization that opposes the death penalty has condemned Alabama authorities' insistence on using an execution method that "continues to go catastrophically wrong."

"In its desperation to execute, Alabama is experimenting on prisoners behind closed doors," Reprieve US Forensic Justice Initiative Director Maya Foa said in a statement.

The use of lethal injection has come under criticism in the United States after some failed cases that caused suffering to the convicts.

In 2018, Alabama approved nitrogen hypoxia as a method of execution, which consists of oxygen deprivation until death by asphyxiation.

Proponents of this method, which has never been used in the United States, say that nitrogen causes an immediate loss of consciousness and that death occurs without suffering.

Alabama law provides a deadline for the convict to opt for nitrogen hypoxia, and Miller's defense said that the inmate expressed that will, but that prison officials missed the relevant statement.

Based on his testimony, an appellate court prevented Miller's execution by lethal injection, but this decision was overturned by the Supreme Court by one dissenting vote.

A spokesman for the Alabama governor, quoted by the local press, said Ivey "anticipates that the execution will resume at the earliest opportunity." (Lusa/RTP)